Gemstone Info

Actinolite Before 1968, actinolite had never been found in a transparent variety suitable for cutting. Although transparent crystals had been found, they were mainly fibrous and very brittle, breaking easily across the prism. In 1968 transparent gem actinolite was found in a crystal form, with sufficient transparency and purity to facet. Actinolite crystallizes in a hexagonal system. The refractive indices are 1.618 – 1.641 with the double refraction of 0.023.It is optically negative in sign. The specific gravity is 3.0-3.2.The chemical formula for actinolite is Ca2 (Mg, Fe)5 (OH)2 (SI4O11)2  with the presence of iron altering the formula to  Ca2 Fe5 (Oh)2  (Si4O11)2.  The colour can range from bright-green to grayish-green. The stones listed are somewhere between the two. Actinolite owes its green colour to ferrous iron.  Pleochroism is distinct, increasing with the amount of iron present, and the more iron present, the darker the colour. The only known locality for this gem variety is Tanzania. The largest stone I found was a fine green actinolite gem, 2.81ct oval in shape, 12 x 7.5mm clean.

Alexandrite is an important variety of chrysoberyl. The stone was named after Czar Alexander II of Russia, where the first chrysoberyls to exhibit a colour change were found in the Ural Mountains near the Telowaja River, east of Ekatarinberg in 1830.The stones, although small, displayed strong dichroism, appearing blue- green in daylight and d-brown in artificial light.Alexandrite, without doubt, owed its early popularity and use as a gemstone to the Russian royal family, who used it extensively in their jewellery. Although alexandrites have been found in many countries since then, the most highly prized of all alexandrites are the Russian variety.  Other countries where alexandrites have been found are Ceylon (Sri Lanka),Czechoslovakia, Burma (Myanmar), Brazil, Southern Rhodesia (Zimbabwe), Madagascar  and most recently, Tanzania. Ideally the stone is a grass-green colour in daylight and columbine red in artificial light. Alexandrite crystallizes in the orthorhombic system with spectacular crystals displaying both contact and interpenetrant twinning often repeating to form pseudo-hexagonal crystals. The green colour in Alexandrite is due to chromium.This is a trace element and is not seen in the chemical formular, which is  Be Al2O4.The colour change in alexandrite is due to the fact that in daylight, a blue light, the blue-green part of the spectrum is emphasized and the stone appears to be green.In tungsten light,the emphasis is then on the red part of the spectrum, and the stone appears to be red. This is helped by the strong dichroism that is found in alexandrite. With a hardness of 8.5 alexandrite was certainly a useful gemstone for jewellery when it was readily available.Owing to its great scarcity today, however, it is only a collector’s item.The refractive indices of alexandrite are 1.747-1.75,  and its specific gravity is 3.5–3.84

Amazonite, a green feldspar mineral,is usually slightly harder then orthoclase and may reach nearly 6 ½ on Mohs’s scale. The density varies from 2.56 to 2.58,and the material, which is optically negative in sign, has for the greatest and least indices of refraction values of 1.522 and 1.530, values which are slightly higher than for orthoclase. The birefringence is 0.008.Microcline, or rather the amazonite variety, shows no distinctive absorption spectrum. The material usually shows a yellow-green glow under long wave ultra-violet light, is inert under a short–wave lamp, and exhibits a weak green glow with a fairly long afterglow after irradiation with X-rays.The green microcline feldspar owes its varietal name, amazonstone, to the Amazon river, but although the mineral is found in Brazil it is not known to come from the vicinity of the Amazon. Green microcline is found in the pegmatite veins of Sao Miguel de Piracicaba and from Joahyma and in the region of Ferros in the State of Minas Gerais.

Amblygonite The mineral amblygonite is a fluophosphate of aluminium and lithium (LiAL (F,OH) PO4), a mineral which, like topaz, has the fluorine replaceable by the hydroxyl group. Amblygonite forms large but rough crystals of the triclinic system which have a tabular habit and perfect basal cleavage. The hardness is 6 on Mohs scale and for gem material the density is between 3.015 and 3.033. The refractive indices for the cut  stone so far examined approximate to 1.611 and 1.637 for the principal rays, the biaxial positive birefringence being 0.026. No clear absorption spectrum is seen with yellow amblygonite and under the long-wave ultra-violet lamp the stones show a very weak greenish glow, but under x-rays the glow is a bright greenish-white. Apart from the physical and optical properties identifying amblygonite, microchemical tests will prove the phosphate content. Powder ‘sneaked’ from the edge of the stone will allow this test to be done and will also allow flame tests to be carried out, for the powder introduced by means of a platinum wire into the colorless Bunsen flame will colour the flame with the characteristic red of lithium.

Anatase is one of the polymorphous forms of titanium oxide (TiO2); rutil and brookite are the others.  Anatase is rarely cut as a gemstone; the mineral is brown or blue in colour and crystallizes in the tetragonal system, usually as bipyramids so that the form can be said to be octahedral. It is owing to this that the alternative name octahedrite is sometime used for the mineral. Anatase has a harness of 5½-6 on the Mohs scale, a density which lies between 3.82 and 3.95, and the refractive indices are 2.493 for the extraordinary ray and 2.554 for the ordinary ray. The uniaxial negative birefringence is 0.061. Anatase shows no luminescence. The mineral results from the alteration of other titanium-bearing minerals. It can be found in veins of hydrothermal origin. Anatase is found in Switzerland.

Andalusite is not commonly met with in jewellery. Andalusite is named after the Spanish province of Andalusia, where the mineral was first found. The stones vary from a greenish-brown, brownish-green to a rich green in colour, the stones owing much of their attractiveness to the extremely pronounced pleochroism which they exhibit. The mineral is an aluminium silicate corresponding to the formula Al2 Si05   and is therefore dimorphous (that is, it has the same chemical composition) with mineral kyanite and fibrolite.  The crystals, belonging to the orthorhombic system, have a prismatic habit with vertically striated prisms which are nearly square in section and capped with pyramids, but much of the gem material is found as water-worn pebbles. The hardness of andalusite is 7½ on the Mohs’s scale and the specific gravity varies from 3.15 to 3.17. The least and greatest refractive indices vary from 1.634 to 1.641 and 1.644 to 1.648,  the biaxial double refraction varying from  0.007 to 0.011. The sign is negative. The dispersion is approximately 0.016 between the B to G interval, and the three pleochroic colours of the stones are yellow, green and red which may variable in intensity. The absorption spectrum of andalusite varies in that the  deep green variety,  believed to come from Brazil, shows a ‘rare earth’ spectrum, with an absorption band, graded in intensity, which ends in a knife-shape edge at 5535Å and which is followed by fine lines at 5505, 5475Å and fainter ones at 5180 and 4950Å. There is a strong absorption in the blue and violet but a band at 4550Å can just be seen. This is noted in the normal types of gem andalusite, and these stones, particularly those from Ceylon, show this band to be accompanied by a narrow band at 4360Å. These last two bands are probably due to iron. Andalusite exhibits no luminescence under the long-wave lamp, but the brownish-green stones from Brazil often show a dark green to yellowish-green glow under the short-wave ultra-violet lamp and greenish- yellow glow under x-rays

Anhydrite is anhydrous calcium sulphate (CaS04). It crystallizes in the orthorhombic system.The hardness is 3-3½ on the Mohs scale. The refractive index is 1.567–1.574. The birefringence is 0.0042–0.044. The specific gravity is 2.89 and is optically positive in sign. The main location where it is found is Simpon Tunnel, Switzerland.

Aragonite is closely related to calcite, and is distinguished by a higher specific gravity and an absence of rhombohendral cleavage. Even under ordinary conditions it is rarely unstable. The colour ranges from yellow to green and violet. A calcium carbonate Ca CO3,Crystallizing in the orthorohombic system, acicular stones of Aragonite are rare. Hardness is 3.50 – 4.00 on the Mohs scale and  the specific gravity is 2.93 – 2.95. The refractive indices are 1.530,  1.680 and 1.685.Aragonite is much less common than  calcite.It is formed under a much narrower range of conditions than calcite and, being less stable, often changes to calcite, with a change in surrounding conditions.

Augite is aluminous pyroxene.Its composition is chiefly CaMgSi2O6 with (Mg, Fe) (Al, Fe) 2 SiO6,and occasionally  also containing alkalis and then graduating toward acmite. Titanium is also sometimes present.  The crystal system is monoclinic.It includes the greenish or brownish black and black types, and occurrs mostly in eruptine rocks.It is usually in short prismatic crystals, thick and stout, or in tabular form. The hardness is 5 to 6½ on Mohs scale. The density is between 3.20 and 3.60. Refractive indices are variable: 1.69 – 1.71. When it is rich in iron or titanium,it becomes distinctly pleochroic. The largest cut stone I had was a black one weighing 40.32cts, from Australia.

Axinite The broad acute-edged triclinic of axinite, which forms such attractive crystal groups, may be found large enough and clear enough to cut into gemstones for those who crave the unusual.Axinite, named after the axe-like shape of the crystals,  is usually a typical clove-brown colour or more rarely honey-yellow or plum-blue.The stones are characterized by the strong dichroism they exhibit,  the twin colours being olive-green, violet-blue and cinnamon-brown for the three principal directions.  The  indices of refraction, biaxial and negative in sign,  are approximately 1.685 for the greatest and 1.675 for the least, the birefingences being 0.010 – 0.012.   The hardness of axinite is 7 on Mohs scale and the density varies from 3.27 – 3.29.The absorption spectrum of axinite shows three fairly obvious bands, a narrow band in the blue-green at 5120 Å and two broader bands at 4920Å and 4660Å in the blue.   Other bands may be observed in some specimens, at 5320Å, 4440Å, and 4150Å, the latter bands being quite strong in some stones.   No luminescence has been observed in gem axinite, but it has been reported that honey-yellow crystal from Franklin, New Jersey, U.S.A. may fluoresce red under short-wave ultraviolet radiation. This may well be due to the included traces of manganese, as so many minerals from the noted zinc ore mining locality behave similarly.Axinite is a complex calcium aluminum boro–silicate (Ca,Mn,Fe,Mg) 3Al2BSi4O15 (OH)), but much variation of the composition occurs owing to the replacement of the calcium by iron, and manganese.  Axinite is found in magnificent crystal groups in Mexico, California, Sri  Lanka, and in the French Alps. Deposits of massive material are located in Luning, Nevada, Pennsylvania, and New Jersey. Other sources are Cornwall, England, Finland, Norway, Germany, Japan, Russia, and Tasmania.

Barite The barium sulphate mineral (BaSO4) barite is also known as heavy spar. The mineral forms orthorhombic crystals which are white in color and may be transparent to opaque. Crystals coloured yellow green, red, blue or brown are sometimes found. The massive white material resembles marble. Barite is rarely cut as a gemstone, except for those who desire anything unusual, for the hardness is only 3 on the Mohs’s scale, which is too low for durability. There is however, a stalactite variety of brown color which shows a concentric structure when cut across the stalactite, and such material has been polished as an ornamental stone.  Barite has a density of 4.3 – 4.6: for gem material the values are near 4.47, and the refractive index are 1.636 1.637 and 1.648. The refraction is biaxial and positive in sign. The crystals show perfect cleavage in two directions. The mineral sometimes fluoresces and often phosphoresces with a faint blue or light green colour under ultra-violet light. Barite can be found world-wide.

Benitoite, a lovely sapphire-blue in colour, is unique in that it is in a class of the trigonal system which has a trigonal axis of symmetry at right angles to it, a class known as the dihexagonal dipyramidal. It is a form that was mathematically considered possible in nature but not found until the discovery of benitoite which is, so far, the only example in nature.The mineral is a barium titanium silicate with the formula BaTiSi 3O9 and thus it has a chemical similarity to sphene. The density lies between 3.65 and 3.68, but the latter value is more common. The hardness is about 6½ on the Mohs scale. The indices are 1.757 for the ordinary ray and 1.804 for the extraordinary ray, the refraction being uniaxial and positive in sign. The birefringence is strong,  being 0.047. The pronounced dichroism shows twin colours of blue for the extraordinary ray and colourless for the ordinary ray, so that to obtain the best colour the table facet should be cut parallel to the vertical crystal axis, and as most of the crystals have a tabular habit, this precludes the cutting of large stones. Benitoite has exceptionally strong fire; the colour dispersion for the B to G interval is 0.039 for the ordinary ray and 0.946 for the extraordinary ray (as measured by C.J.Payne).  Thus the fire approximates to that found in diamonds, but the effect is masked by the body colour of the stone. The lower density allows distinction from sapphire, but if the stone is set, determination of this constant would not be possible and a refractometer reading would be necessary. Care is needed, however, in observing the shadow edges, as the edge of the extrodinary  ray at its highest is in the region of the shadow edge of the contact liquid (1.81) and may be missed unless its movement is carefully noted. The pronounced dichroism may also give information of value, for there is no colourless ray in a sapphire.  What may be a simple and more easily performed test is given by the bright fluorescent glow seen under short-wave (2537Å) ultra-violet light, or under x-rays. This blue glow is not observed under long-wave (3650Å) radiation. Benitoite has been found in only one locality, in the Diablo Range of the California Coast Ranges (part of the larger Pacific Coast Ranges) in San Benito County, where the flattened triangular shape crystals, never of large size, are found in association with anther rare titanium mineral, neptunite, in the matrix of white natrolite.

Beryl Maxixe A deep blue beryl of a type was recently mentioned by Mr A. E. Farn as being of a blue more reminiscent of Ceylon sapphire than that of aquamarine. And, as far as colour alone is concerned that description is true of the stone. These seemed to be no trace of green in its visual appearance, only blue of near sapphire colour.Optical examination under the microscope disclosed very little in the way of inclusions. These were only a couple of short, rod like cavities, quite invisible to the naked eye; otherwise the stones were remarkably clean. To determine refractive index aquamarines (from Minas Gerais, Brazil) was made, because it will be seen that the two sets of figures differ quite widely, and yet despite that, they appear to have an exactly similar absorption spectrum. Exhibiting a greenish glow reminiscent of a similar fluorescence seen in synthetic Sapphire under the same radiation, but lighter in colour, and more easily seen. It is know that the blue of synthetic sapphire is obtained by the addition of titanium and iron to the boule (though the later elements appears to evaporate in the process of production). May it therefore not be possible, in view of the similar fluorescence’s, that the colour of blue beryl is also due to traces of titanium. The suggestion is purely confectual. At all events, the absence of iron is evidenced by the fact that beryl does, indeed, fluoresce. The absorption spectrum was, perhaps, the most interesting feature of all, and was at first glance, strongly reminiscent of a spectrum due to the presence of chromium. These was a very strong, though not very clear cut line in the red, the measurement was 6970Å; this agrees with the figure given by B. W. Anderson for the equivalent line in the spectrum of Maxixe aquamarine. Close examination of this line showed that it was not a doublet (ie two very fine lines close together), whereas the corresponding line in stones whose colour is due to chromium is always such. Besides the strong line there were also two much fainter lines fairly close to it on the short wave side again some what resembling a chromium spectrum. These last two lines could be seen through a prism spectroscope. They were two weak too be seen and measured by my” wavelength” instrument. On rotating the stone on the table of the refractometer, there was no detectable movement of either of the two shadow edges, thus indicating that, as shown in the figure, the stone has been cut from its crystal with its table across the prism, ie at right angles to the optic axial direction. Further conformation of this orientation was afforded by the fact that through the table dichroism waspractically nonexistent.

Beryl Morganite The lovely pink, rose and peach–coloured beryl called morganite (after  J. P .Morgan), owes its shade of colour to the presence of a trace of oxide of lithium. The alkali metals, caesium and rubidium are often, by small scale replacement, impurities in the pink beryl and tend to raise the density and refractive indices. Most, but not all pink beryls have a density of 2.80 to 2.90 although some may be as low as 2.71.  The refractive indices lie between 1.580 and 1.590 for the ordinary ray and the double refraction  varies between 0.008 and 0.009.   A few pink beryls (those with lower density) have refractive indices only a little higher that that of aquamarine. Some colourless beryl (rich in caesium) also have this higher density and indices of refraction.  The dichroism of morganite is distinct, the twin colors being a pale pink and a deeper bluish pink, the strong colour being that of the ordinary ray. There is no characteristic absorption spectrum. The luminescence under ultra-violet light is a weak lilac, but under x-rays is an intense, but not bright, crimson glow. A pure-pink–coloured beryl is found in the state of Minas Gerais in Brazil.

Boracite Some faceted stones of a pale green colour have been cut for collectors from the magnesium chloro-borate mineral known as boracite. The chemical formula for the mineral is Mg6 Cl2 B14O26  and it forms crystals of the cubic system with cubic, tetrahedral or dodecahedral habit. The hardness is 7 on the Mohs’s scale, the density is 2.96 and the refractive index is 1.661 to 1.671. The mineral is pseudo-isometric and may be orthorhombic so that some double refraction of the positive sign may be apparent. The luster is vitreous to adamantine and a very weak greenish fluorescence may be seen. The mineral is found in Germany, and also Bolivia.

Brazilianite In 1944, some yellowish-green crystals were found in a pegmatite dyke at a locality near Conselheira Pena in Minas Gerais, Brazil. When first found, these crystals were thought to be  chrysoberyl, to which they have some resemblance in colour but not in crystal form. Some of the crystals were then shown by a dealer to my friend F. H. Pough, then in Brazil, and he at once determined the differences in crystal form and a lower hardness than for chrysoberyl but he could not recognize the crystals as being of any mineral he knew. Later,in conjunction with E. P.Henderson, the crystals were found to be those of a completely new mineral, to which the name brazilianite was given in honor of the country in which they were found. Brazilianite is a hydrous sodium aluminum phosphate and has the formula Al3 Na (Po4) (OH)4.:Thus,it has some degree of chemical affinity to turquoise and amblygonite. The crystals, generally prismatic in form, are usually of fair size belonging to the monoclinic system of crystallization. The cleavage is perfect in onedirection, which is parallel to one of the pinacoid faces. The material is brittle and shows a conchoidal fracture. The hardness is low, being 5½ on Mohs scale, and the density varies between 2.980 and 2.995. The optical properties show brazilianite to be biaxial positive with indices of refraction of 1.603, 1.612 and 1.623, giving a double refraction of 0.020. The dispersion is low, being only 0.014 for the B to G interval. There is no luminescence when the stone is bathed in ultra-violet light and no  definite absorption spectrum has been noted. The dichroism is weak, merely a slight change of color being noted. This was the largest known at the time. Some years ago in the late 60s I acquired a large brazilianite gem stone, with a trillion cut  of 110 cts,  from Dr. Herman Bank in Idar-Oberstine, West Germany.

Cassiterite Tin oxide (SnO2) is the principal ore of tin. . Usually owing to the presence of iron, the mineral is black and opaque, but occasionally crystals occur which  are light reddish–brown in colour and sufficiently transparent to cut as faceted stone, their interest is to collectors only. The mineral crystallizes in the tetragonal system as square-section prismatic crystals capped by pyramids, and the crystals are often twinned with twin planes parallel to one of the pyramid faces producing a knee-shaped form (geniculate twins) The crystals show only an indistinct cleavage. Cassiterite has a hardness of about 6½ on Mohs scale and is characterized by having a high specific gravity of 6.8.  The refraction, uniaxial-positive, has indices of 2.003 for the ordinary ray and 2.101 for the extraordinary ray, thus the birefringence  is 0.098.  The dispersion is 0.071 between the B to G Interval and the luster shown by the minerals is adamantine. Due probably to the iron content, cassiterite does not show any luminescence, and the stone shows no distinctive absorption spectrum. Cassiterite can be distinguished from brown diamond, brown zircon and sphene by the great density of the mineral, from zircon by the absence of any typical absorption lines, from sphene by the absence of a much weaker dichroism, and from diamond  by the double refraction. The largest cassiterite I had was a yellow-brown emerald cut stone 19.74cts. It was in the – Precious  Gemstones from the Max Davis Collection Copyright 1968.

Chambersite it crystallizes in the orthorhombic system. The crystal are found very small. The color is white to violet. Its chemical composition is 4(Mn3 B7O13 CL). The hardness id 7 on the Mohs’s scale, and it specific gravity is 3.49. The refractive indices are 1.732, 1.737 and 1.744, the double refraction being 0.012. Chambersite is found in U.S.A.

Clinozoisite is an epidote containing less than 10 percent of the iron molecule and is usually a much lighter green in colour.The mineral is monoclinic and has a density near 3.37.The refractive indices are 1.724 and 1.734, with a birefringence of 0.010,which,however, is positive in sign. The absorption spectrum is similar to that of a normal epidote. The hardness is 6½ on the Mohs scale.When manganese is a replacing metal in the epidote composition, the mineral piedmontite is produced. This is a red mineral which does not have gem significance, but is the cause of the red colour in the red porphyry in Egypt.  A chrome epidote of tawnamite is a deep green stone showing strong pleochroism (emerald–green and bright yellow), which is found at Tawmaw in the Kachin Hills of Upper Burma. Clinozisite is found in America.

Demantoid There was a new find of Demantoid in Turkey. The geologist obtained some rough and cut samples of this material in time for the 2008 Tucson gem shows, and loaned several pieces to GIA for examination. The geologists supplier, also exhibited the garnet in Tucson, in both 2008 and 2009. He informed us that he owned the claim, located near Erzincan, 700 km east of Ankara. Due to the long and severe winters, people mine the area for only three months of the year (June through August). Using a pneumatic hammer and hand tools, they produced ~300 g of cuttable material in 2007, and ~120 g in 2008, although most was fairly small and included. To date, they have had about 20 stones cut (in Germany) that range from 0.5 to 1.2 ct, as well as an abundance of melee-sized material, totaling ~300 ct. A gemological examination of 13 stones that they obtained showed that six were Andradite (including some Demantoid) and seven were grossular (Tsavorite). The presence of grossular could not be explained, and Mr. Özütemiz indicated that those stones may have been introduced into the parcel at the cutting factory. The six andradite samples (figure 1; 0.15–0.38 ct) were characterized for this report: color—orangy brown, brownish yellow, yellow-green, and yellowish green (the four yellow-green to yellowish green samples were demantoid); RI—over the limits of the refractometer; hydrostatic SG—3.82–3.93 (brown samples) and 4.17–4.43 (green samples); inert to both long- and short-wave UV radiation; and a 440 nm cutoff visible with the desk-model spectroscope. Microscopic examination revealed radiating curved fibrous needles with associated fractures (some of which were filled with a transparent filler or partially dried residue), blocky “fingerprints,” and cubic growth zoning. These properties are typical for andradite (e.g., M. O’Donoghue, Ed., Gems, 6th ed., Butterworth-Heinemann, Oxford, UK, 2006, pp. 206–210). In addition, demantoid samples had distinct brown and green cubic color zoning, similar to that seen previously in two samples of andradite from Iran (see Spring 2007 GNI, pp. 65–67). Quantitative chemical analysis of the six andradites was performed by electron microprobe at the University of Oklahoma (see G&G Data Depository), and revealed the garnets contained 93.4–98.9 mol% andradite component. The demantoid samples contained significant levels of Cr, while the orangy brown andradite had the highest Ti content. Alethea Inns (aletheainns@gia.edu) GIA Laboratory, Carlsbad Brendan M. Laurs Figure 1. These andradites (0.15–0.38 ct) were reportedly cut from rough produced in eastern Turkey in late 2007. From left to right, the samples are 5a, 5b, G, E, F, D. Photo by Kevin Schumacher. G&G Data Depository: Electron microprobe data to accompany A. Inns, “Gem News International: Andradite from Erzincan, eastern Turkey,” Summer 2009 G&G.

Demantoid Garnet is an incredibly beautiful gemstone. The demantoid garnet displays more flash and fire than other members of the garnet family. The important green andradite garnet is called demantoid, so named from its adamantine luster. It suffers from the defect of the low hardness of andradite, which is only 6½ on Mohs scale. The density varies from 3.82 to 3.85 and the single refractive index from 1.888 to 1.889. The colour dispersion in demantoid is 0.037 for the B to G interval. This gives the stone a greater fire than a diamond, but the effect is to some extent masked by the body colour of the stone. Demantoid garnets appear reddish when viewed through the Chelsea colour filter, and the stones are characterized by an absorption spectrum consisting of a strong band at 4430, which generally appears as cut off owing to the general obscurity of the violet end of the spectrum and only in the paler stones can any violet light be seen beyond this band. The band is due to iron (ferric oxide) which to some extent gives the green colour to the stone, but iron greens are dull greens and the bright green of demantoid is caused, as mentioned earlier, by the trace of chromium. Therefore, in fine demantoids a chromium absorption spectrum may be seen, the lines being the strong doublet near 7010, a weak but sharp line at 6930 and two bands in the orange at 6400 and 6220 which are vaguer and broader than usual in chromium spectra. Demantoid does not exhibit any luminescence under ultra-violet light or x–rays. Demantoid garnet was discovered during the mid-1800s in Russia’s Ural Mountains. The beautiful fire and brilliance of this gemstone made it a stone that quickly became a favorite of Russia’s elite. Then, World War I began, and only a few Russian demantoid garnets were found after the conflict. There are three kinds of green garnets: tsavorite garnet, uvarovite garnet, and demantoid garnet. Demantoid are found in Russia, Italy, and Africa.

Diamond The atomic structure of a diamond is of considerable interest, for each carbon atom lies at the center of a regular tetrahedron, and is joined to the four other carbon atoms which lie at the corners of the tetrahedron. The structure is therefore a system of interlacing hexagons with each atom linked (covalently) to four others. It is owing to the nearness of the atoms, 1.54Å and the fact that the whole crystal is, in effect,  one molecule, that any breakdown of the structure involves the breaking of the strong atomic bonds. Hence, the chemical stability and hardness of diamond, the hardest mineral known,  with the symbol 10 on Mohs scale of hardness.  Despite its hardness, diamond has four directions of weakness along which the crystal will split fairly easily,  leaving smooth surfaces. The cleavage is, in the case of diamond, parallel to the faces of the octahedron. In such a direction the atomic bonds are fewer than in other directions, hence the cohesion is weaker along these planes. The density of diamond is 3.52 and owing to the purity of the crystal it varies little from the value. The exception is in the case of the crystalline variety known as carbonado which can have a much lower value of density, values from 3.5 to as low as 2.90 having been recorded for this material. In diamond the red ray (with a wavelength of 6870Å) has a refractive index of 2.0407 the violet ray (wavelength of 3970Å) has an index of 2.465.The violet ray is always bent more than the other colours of the spectrum when entering an optically denser substance. A measure of the dispersion is the difference between the refractive indices of the red and the violet rays, and for diamond this is 0.058.The dispersion of diamond is B to G 0.44.

Dunilite Long mistaken for green peridot,dunilite was identified in 2000 as a new gem variety from the olivine group.It is an orthorhombic, made of magnesium,iron, and manganese silicate, though the x-ray diffraction pattern shows that the line spacing is similar to peridot. These spacings, however,are not identical with those for peridot (Dr Mary L Johnson,GIA,July 8 1998).The colour of dunilite varies from light greenish yellow to brownish green, the depth of colour being related to the iron content. Dunilite is not quite as hard as quartz and thus is not one of the more durable gems. It does not however, have cleavage, and offers no problems for the lapidary.Faceted stones have a light greenish yellow appearance resembling yellow chrysoberyl or sinhalite. The usual size range is from 0.05 to 1 carat. Tiny faceted dunilites are cut from the rough, transparent crystals. The hardness is 6.50, it is from the olivine group, and the crystal system is orthorhombic. The chemical composition is (Ca, Na) 2 (Al, Mg, Fe2+ Fe+3)6 (Si, B, Al) 6 O20.  The refractive index is a=1.677, b=1.700 y = 1.18. The birefringence is 0.041 and the specific gravity is 3.48. Locality: Top of Katukubura Mountain,Kolonne, Sri Lanka.

Epidote The dark brown-green or pistachio-green color of epidote is characteristic of the mineral. Closely related to zoisite, epidote is a calcium aluminium silicate with some iron, which has the formula Ca2 (Al oh) Al2 (SiO4)3. The mineral crystallizes in the monoclinic system, usually as deep vertically striated prismatic crystals of a peculiar greenish-brown colour, which is darker the greater the percentage of iron they contain. The hardness is 6½ on the Mohs scale, the density being about 3.4 and the principal refractive index are 1,736 and 1.770 with a biaxial double refraction of 0.034 which is negative in sign. With less iron the refraction indices and the amount of double refraction can be lower than the values given above. The stones are strongly pleochroic, the colours being green, brown and yellow. The fire is moderate, the dispersion being 0.030, and the lustre is vitreous to metallic. The epidote is found in Austria and Norway and other locations.

Euclase Euclase owes its name to the strong cleavage which make the cutting of crystal such a hazardous matter and the cut stones so liable to fracture.Euclase is a beryllium silicate (Be(Al,OH)Si04) and forms monoclinic crystals characterized by numerous smooth faces. The color of the material is usually a pale aquamarine although stones with a strong greenish-blue color have been encountered. Like aquamarine in appearance, euclase may be distinguished by the positive biaxial optical character and the refractive indices for the principal rays, 1.652 and 1.672 with a birefringence of 1.020.The density is fairly constant at 3.10, and the hardness is 7 ½ on Mohs scale. The dispersion is 0.016 for the B to G interval,  which is too weak for the stone to show fire. The absorption spectrum is, in the case of the deeper coloured stones, characterized by two vague bands in the blue at 4680Å and 4550Å and also the line at 7050Å.  The luminescence, even under X-rays is too feeble to be of value in distinction. Euclase shows a similarity to green spodumine both in color and properties; sudumene, however has less birefringence (0.015) and a slightly higher density (3.18). The principal locality for euclase is the Duro Ptìreto region of the Brazilian State of Minas Gerais. My largest euclase was a blue-green stone, which weighed 9.10 cts (1968).

Garnet Demantoid There was a new find of Demantoid in Turkey. The geologist obtained some rough and cut samples of this material in time for the 2008 Tucson gem shows, and loaned several pieces to GIA for examination. The geologists supplier, also exhibited the garnet in Tucson, in both 2008 and 2009. He informed us that he owned the claim, located near Erzincan, 700 km east of Ankara. Due to the long and severe winters, people mine the area for only three months of the year (June through August). Using a pneumatic hammer and hand tools, they produced ~300 g of cuttable material in 2007, and ~120 g in 2008, although most was fairly small and included. To date, they have had about 20 stones cut (in Germany) that range from 0.5 to 1.2 ct, as well as an abundance of melee-sized material, totaling ~300 ct. A gemological examination of 13 stones that they obtained showed that six were Andradite (including some Demantoid) and seven were grossular (Tsavorite). The presence of grossular could not be explained, and Mr. Özütemiz indicated that those stones may have been introduced into the parcel at the cutting factory. The six andradite samples (figure 1; 0.15–0.38 ct) were characterized for this report: color—orangy brown, brownish yellow, yellow-green, and yellowish green (the four yellow-green to yellowish green samples were demantoid); RI—over the limits of the refractometer; hydrostatic SG—3.82–3.93 (brown samples) and 4.17–4.43 (green samples); inert to both long- and short-wave UV radiation; and a 440 nm cutoff visible with the desk-model spectroscope. Microscopic examination revealed radiating curved fibrous needles with associated fractures (some of which were filled with a transparent filler or partially dried residue), blocky “fingerprints,” and cubic growth zoning. These properties are typical for andradite (e.g., M. O’Donoghue, Ed., Gems, 6th ed., Butterworth-Heinemann, Oxford, UK, 2006, pp. 206–210). In addition, demantoid samples had distinct brown and green cubic color zoning, similar to that seen previously in two samples of andradite from Iran (see Spring 2007 GNI, pp. 65–67). Quantitative chemical analysis of the six andradites was performed by electron microprobe at the University of Oklahoma (see G&G Data Depository), and revealed the garnets contained 93.4–98.9 mol% andradite component. The demantoid samples contained significant levels of Cr, while the orangy brown andradite had the highest Ti content. These andradites (0.15–0.38 ct) were reportedly cut from rough produced in eastern Turkey in late 2007. From left to right, the samples are 5a, 5b, G, E, F, D. Photo by Kevin Schumacher. G&G Data Depository: Electron microprobe data to accompany A. Inns, “Gem News International: Andradite from Erzincan, eastern Turkey,” Summer 2009 G&G.

Hambergite Hambergite is a beryllium borate (Be2 (OH) BO3), and forms colourless prismatic crystals belonging to the orthorhombic system. A discovery has now been made of colourless crystals in central Madagascar. The mineral is characterized by its large double refraction which is biaxial and of positive sign. Indices range from 1.553 to 1.559 and from 1.625 to 1.631 for the principal rays; the birefringences are 0.072. The lustre is vitreous and the colour dispersion is small (0.015 B to G). In relation to the refractive indices, the density of hambergite is remarkably low being only 2.35. The hardness is 7½ the Mohs Scale, and no luminescence was detected in the specimens examined. Hambergite was named after Axel Hamberg.

Iolite Owing to the sapphire–blue color of gem-quality iolite, the material was, in the earlier days, misnamed “water sapphire”. Iolite is a complex silicate of magnesium and aluminium (Mg2 AL4 Si5 O18) but replacement of part of the magnesium by ferrous iron and manganese, and the aluminium by ferric iron, often occurs. These replacements make the constants for iolite somewhat variable. Iolite crystallizes in the orthorhombic system. The crystals are often found as short pseudo-hexagonal prismatic twins, which to some extent resemble quartz. Most of the gem material is found in gravel as water-worn pebbles. The mineral has a hardness of 7-7½ on the Mohs scale. The density may very from 2.57 to 2.66, but most gem material is confined to more restricted range of 2.57 to 2.61. The indices of refraction may vary within the limits of 1.53 – 1.54 and 1.54 and 1.54 -1.55 and the amount of birefringence varies from 0.008 to 0.012. The sign of refraction is negative but it has been reported as positive.   Iolite has a weak dispersion, the value being 0.017 in the B to G interval.The pleochroism of iolite is particularly strong, the colors for the three principal directions being yellow, light blue and dark violet-blue. The best blue colour is seen when the crystal is viewed down the length of the prism. The absorption spectrum of iolite resembles that of blue spinel in that the spectrum is due mainly to ferrous iron. The strong dichroism of iolite precludes any confusion with the singly refracting spinel. The absorption bands are not strong and are 6450, 5930, 5850, 5350, 4920, 4560, 4360 and 4260Å. The absorption spectrum varies with the direction so that in the direction of the blue-violet colour the 6450 and4260 bands are masked by the general absorption and appears as cut-offs which shorten the spectrum. Due probably to the iron content, iolite does not luminesce under ultra-violet light. Iolite has been found in Madras, India, Tanzania, and in Finland.

Maw-sit-sit is one of the more unusual gemstones in the world, from its curious name to its variable chemical composition. It is a recent find, first identified in 1963 by the late Swiss gemologist Dr.Edward Gubelin. It was named after the village in northwestern Burma which is close to the site where it was first found. Typically maw-sit-sit is green with distinctive dark-green to black veining. Maw-sit-sit is considered to be a rock rather than a mineral, since it is composed of a number of different minerals, including kosmochlor (a mineral related to jadeite) and varying amounts of jadeite and albite feldspar. It is often classified as a member of the jade family but it is not really a variety of jadeite. Sometimes maw-sit-sit is called a cousin of jade, which seems appropriate.The source location for maw-sit-sit lies in the foothills of the Himalayas, near the historic imperial jadeite mines of northern Burma. Thus far maw-sit-sit has only been found in this single location, making it a rare material indeed.The dominant mineral species in maw-sit-sit is kosmochlor, a sodium chromium pyroxene. About 60% of maw-sit-sit is made up of osmochlor, which is the primary component of both the brilliant emerald green and the dark green-black patches. The other major component is a chromium-enriched jadeite, making up about 15 percent of the total material. Kosmochlor, once known as ureyite, also has an unusual name. It means green from outer space, since the mineral was found originally in meteorites. Maw-sit-sit has a refractive index ranging from 1.52 to 1.74, depending on its exact chemical composition. The lower end of the range is most common. It has a hardness rating of 6 to 7 on the Mohs scale, and a density or specific gravity ranging from 2.5 to 3.5. Maw-sit-sit is translucent to opaque and is almost always cut as cabochons or carved.

Opal Honey Color Catseye  Not too long ago, we examined in the New York laboratory a rough specimen of banded, translucent, and brownish to green material that we tentatively identified as common opal, with no play of color. Later, we were allowed to examine and photograph a 1.5-ct chatoyant orange-brown cabochon, said to have been cut from a clear band of this material. Testing by X-ray diffraction in the Santa Monica lab established the presence of cristobalite. A diffraction pattern superimposed on an amorphous background indicates that the material is opal. This is the first cat’s-eye opal of this type seen by the lab.

Painite During 1957 a deep garnet-red crystal emanating from the Mogok gravels at Ohngaing village, Upper Burma, was identified as a new mineral, a mineral which could be cut as a gemstone. There may, therefore, be cut gemstones in existence which at present are unidentified.  Painite, a calcium boro-silico aluminate, belongs to the hexagonal crystal system. The hardness of the mineral is 7½ on the Mohs scale and the density is 4.01. The refractive indices are for ordinary rays 1.816 and for the extraordinary rays 1.787; the mineral is therefore uniaxial.  The colours are ruby-red and pale brownish orange, the deeper colour being that of the ordinary ray.   The absorption spectrum shows a faint chromium spectrum. The original specimen showed a red colour under crossed filters, a weak red glow under long-wave ultraviolet light and a stronger red glow under the short-wave lamp. Under x-ray the glow was a very dim bluish-yellow. The crystal showed featherlike sheets of minute cavities and inclusions of a  large hexagonal tabular crystal. This mineral was named after A.C.D. Pain, who found the crystal about 1951.

Phenacite, a gemstone which has occasionally appeared in jewellery, resembles in appearance colorless quartz. Indeed, it was so often mistaken for quartz that the name phenacite is derived from the Greek word that means “to cheat.”  Phenacite is a beryllium silicate (Be2SiO4) and is found in bright crystals of the trigonal system, which may be either tabular or prismatic in habit. These crystals show no distinct cleavage. The refraction is uniaxial and positive in sign, the indices being 1.654 for the ordinary ray and 1.679 for the extraordinary ray thus the double refraction approaches that of tourmaline, being 0.016.The stones have little fire, for the dispersion is only 0.015 for the B to G interval, the bright vitreous luster being their only saving grace.  Phenacite is harder than quartz at 7½- 8 on the Mohs scale,  and the specific gravity varies between 2.95 and 2.97. There is no characteristic absorption spectrum, and luminescence under ultra-violet light is too weak to be of value – it may be palish green. Under x-rays, the fluorescence is quite distinct and of a blue colour; there is no afterglow. Crystals of a bright wine-yellow and pale rose-red (the latter said to emanate from Russia), have been reported, but such hues have not been seen within cut stones. The usual occurrence of phenacite is most commonly in pegmatites,in granite druses and in mica schists. A prominent occurrence of large colourless crystals is at Minas Gerais, Brazil.

Pollucite is an extremely rare mineral. It is a silicate of aluminium and caesium with the formula H2 Cs4 Al4 (SiO3 )9, and is unique in containing the rare alkali metal caesium as an essential constituent. The mineral was first found in the granite pegmatites of the island of Elba. Breithaupt, in 1846, found pollucite in association with another rare mineral and named them pollux and castor after the twin brothers of Helen of Troy in Greek mythology.Subsequently, the name was modified to pollucite by attaching the usual mineralogical termination. Castor was proved later to be the mineral petalite. Pollucite is a colourless or white mineral which has a vitreous lustre and is found in crystals of the cubic system, usually as cubes; it is often found massive. The hardness is 6½ on the Mohs scale and the mineral has a density range between 2.85 and 2.94 (a colourless specimen weighing 1.725 carats was determined by L.J. Spencer as 2.86). The refractive index varies from about 1.517 to 1.525, the material from main having the higher index. The dispersion is low (0.012 for the B to G interval), and the luminescence under ultra-violet light or x-rays is usually orange or pink in colour.

Sapphirine is a rare mineral, a silicate of magnesium and aluminium with the chemical formula (Mg,Al)8(Al,Si)6O20).Named for its sapphire-like colour, sapphirine is primarily of interest to researchers and collectors, well-formed crystals are treasured and occasionally cut into gemstones. Named for its sapphire-like colour, sapphirine is primarily of interest to researchers and collectors: well-formed crystals are treasured and occasionally cut into gemstones.Typical colours range from light to dark sapphire blue, bluish to brownish green, green, and bluish or greenish gray to black; less common colours include yellow, pale red, and pink to purplish pink. Sapphirine is relatively hard (7.5 on Mohs scale), usually transparent to translucent, with a vitreous lustre.rystallising in the monoclinic system, sapphirine is typically anhedral or granular in habit, but may also be tabular or in aggregates: Twinning is uncommon.Fracture is subconchodial to uneven, and there is one direction of perfect cleavage. The specific gravity of sapphirine is 3.54–3.51, and its streak is white.Sapphirine’s refractive index (as measured by monochromatic sodium light, 589.3 nm) ranges from 1.701 to 1.718 with a birefringence of 0.006–0.007, biaxial negative. Refractive index values may correspond to colour: brownish green specimens will possess the highest values, purplish-pink specimens the lowest, and blue specimens will be intermediate between them. Pleochroism may be extreme, with trichroic colours ranging from: colourless, pale yellow or red; sky to lavender blue, or bluish-green; to dark blue. There is no reaction under ultraviolet light.Sapphirine occurs in a variety of rocks, including granulite and amphibolitefacies,calc-silicate skarns, and quartzites; it is also known from xenoliths. Associated minerals include: calcite, chrysoberyl, cordierite,corundum, garnet, kornerupine, kyanite, phlogopite, scapolite, sillimanite, spinel, and surinamite.

Sinhalite For a number of years a stone understood to be brown peridot has been known. The stone has refractive indices and other properties close to the peridot, but has a somewhat higher density of 3.48, a discrepancy ascribed to an increase of unessential iron. In 1952 the nature of the stone was questioned and by means of  X-ray crystal lographic and chemical analysis, the stone was found to be a completely new mineral species.  It is magnesium aluminium iron borate with the formula Mg (AlFe) BO4 (peridot has the formula (Mg Fe) SiO4). The colour of sinhalite varies from pale-yellow brown to golden or greenish brown to black, the depth of color being in relation to the iron content. The stones have a strong resemblance to brown chrysoberyl or brown zircon.The crystallization of sinhalite is orthormbic, but except for an isolated recovery of good crystal from Burma gem gravels, the material is found only as rolled pebbles from the gem gravels of Ceylon.  Indeed,it is from Sinhala, the Sanskrit name for Ceylon, that the name of the species is derived.The hardness of sinhalite is 6 ½ on the Mohs scale.  The density lies between 3.47 and 3.49 and the refractive indices are 1.67 and 1.71for the principal rays, the negative biaxial birefringence being 0.038.   The pleochroism is distinct with the colours pale brown, greenish–brown and dark brown. The dispersion is 0.018 between the B to G interval, and the absorption spectrum is similar to that of peridot: the bands are at 4930, 4750, 4750, 4630 and 4520?, and there is eneral absorption of the violet.   Sinhalite does not luminesce.Smithsonian Institute 110cts Brown Ceylon (3587) Smithsonian Institute 44cts Brown Ceylon (3548l) Royal Ontario Museum University of Toronto Cinnamon, 52.58 Ceylon Supplied by Max Davis  (Stones) Ltd.

Taaffeite Count Richard Taaffe (1898-1967), an Irish gemologist,  was the discoverer of taaffeite in 1945. This is truly the rarest and most important discovery of its time. The original stone found was only 0.77 cts.  Count Taaffe, a Dublin gemologist, came across a pale mauve cut stone whtch had most of the characteristics of spinel but which showed distinct signs of double refraction. Investigation showed that the stone was a completely new mineral. This was followed by the finding of a second cut stone in 1949. Taaffeite is a beryllium magnesium aluminate (BeMgAl16O32) , and seems to be the only known mineral including both beryllium and magnesium as essential constituents. The chemical composition indicates that the stone is intermediate between spinel and chrysoberyl.  The system of crystallization is hexagonal. The hardness is 8 on Mohs scale: The hardness is 8 -8.5:  the density is from 3.60 to 3.61 and the indices of refraction vary from 1.721 to 1.723 for the ordinary ray and from 1.717 to 1.718 for the extraordinary ray, the mineral having the small negative uniaxial birefringence of 0.004. The stone was too pale for the dichroism to be observed and the same reasoning applies to the absorption spectrum. However, in the original stone found by Count Taaffe, a vague band in the blue was observed very near to and exactly like the band shown in the blue spinel.  A band due to ferrous iron, at 4580 ? the fluorescence shown by these is stones under ultra- violet light, is a distinct green and this is more pronounced under x-rays. It has a luminescent glow similar to that shown by pale mauve spinels. The location of the original taaffeite is unknown but the stones are believed to come from Ceylon, since other stones have been found in Ceylon (now known as Sri Lanka).

Tanzanite Some fine transparent blue-violet crystals, which  were eventually identified as zoisite, had been found in Tanzania near the Kenya border, in the Gerevi Hills.  These blue zoisites are unlike anything so far seen, and did appear a to be really interesting addition to the coloured-stone range. The chemical composition is Ca2Al3(SiO4)3OH.  The values of the refractive index were 1.6917, 1.6927 and 1.7005. The birefringence was thus 0.0088, and strong positive. The density of a number of pieces was determined by hydrostatic weighing in ethylene dibromide, and proved to be remarkably constant at 3.354 – 3.355.  The hardness was 6 or perhaps a little higher. The spectroscope revealed a fairly broad absorption band in the orange-yellow, centred near 5950Å.  The strength and position of the band varied somewhat with direction through the mineral, as might be expected from so pleochroic a substance.  There were two fainter broad bands in the green ( 5280Å) and blue (4550Å).   It was interesting also to detect several narrow lines in the deep red, resembling weak chromium lines. These, and the band’s 5950Å band, are reminiscent of the green vanadium tourmalines, and it seems quite possible that the colour of the blue-violet zoisite is due to traces of vanadium. There was no noticeable fluorescence under long-wave or short wave ultra-violet,  nor under crossed filters  – thus chromium was presumable  absent. Under x– rays there was a feeble bluish glimmer. These new zoisites were named tanzanite.   The stone was discovered in 1967.

Thomsonite Belonging to the zeolite family of minerals, thomsonite is a hydrated calcium sodium aluminium silicate – (Ca, Na2) Al2Si2O8.H2O – which crystallizes in the orthorhombic system. The gem material is mostly columnar with a radial structure, or as radiated spherical aggregates or concretions, which are translucent and closely resemble agate. They may be milk-white, yellow reddish, brownish and greenish, the fibres being arranged radially producing eye-like forms. Thomsonite has a hardness of about 5 on the Mohs scale: the density varies from 2.30 to 2.40, and the refractive indices are also variable and may have limits between 1.52 and 1.54; a vague shadow edge is all that would be seen on a refractometer. The lustre is porcellaneou, and the stones takes a good polish although they are somewhat brittle. Under ultra-violet light the luminescence seen is patchy with brownish and whitish glows. The mineral results from the alteration of feldspars and nephelite.

Thulite is a hydrous calcium aluminium silicate (Ca2 (Al, OH) Al2 (SiO4) 3) and crystallizes in the orthorhombic system. Thulite has a hardness of 6 on the Mohs’s scale,  and the density dose not vary greatly from 3.10. The refractive index, at least so far as the vague shadow edge that can be seen on the refractometer scale shows, is near to 1.70. There is no typical absorption spectrum. Found in Norway.

Violane is a translucent to opaque dark violet-blue variety of diopside, which is usually found in a massive form, but sometimes occurs in a crystalline form. The material, which has a somewhat waxy luster, takes an attractive polish and is worked into beads, but is mostly used for fancy articles and for inlays.   The hardness of violane  is 5 – 6 on Mohs scale.  Its chemical composition is CaMgSi2O6 , the  density is near 3.23,  and the index of refraction, so far as a blurred shadow edge can be read on the refractometer, is 1.69. Violane is found at Saint Marcel, Piedmont, Italy.Its crystal form is monoclinic, usually seen as single, short prismatic crystals.  Crystals may also be somewhat elongated, and usually have good terminations. It also occurs in a massive form, grainy, columnar, bladed, and fibrous, as cleavage fragments, and in disordered aggregates of elongated crystals.

 

 

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