- Varieties and Trade Names of Garnet
- Formation of Garnet
- Sources of Garnet
- Garnet Jewelry
- Physical & Optical Properties of Garnet
- Test and Identification of Garnet
- Valuation of Garnets
The January born person who dislikes red will usually be surprised when he learns from an informed jeweler that his birthstone, garnet, occurs in virtually every color of the spectrum, except blue, and that it is perfectly correct and permissible to wear any of the colors to symbolize his birth date. It can also be a revelation when he is shown by example that even the comparatively common red and purplish-red species can be exceedingly attractive and desirable in their finer qualities.
The name garnet comes from the Latin "granatus," meaning "seedlike" or "having many seeds", because garnet crystals in a rock reminded early scientists of the shape and color of pomegranate seeds (Latin, "granatum" ).
In vedic sastras it was classified as "Tam da". Garnet was known thousands of years before the Christian era and is mentioned in early Biblical writings, where it was probably also called both ruby and carbuncle. The word carbuncle is from the Latin "carbunculus," meaning "little spark"; in Pliny's time all glowing red gems were referred to by this name. Today, the term is applied only to red garnets cut in the cabochon form. Not only was garnet considered the gem of faith, constancy and truth, but it was believed to have curative powers. One of the most common practices was to grind it into powder and use it as a poultice. In this form, red garnet was said to relieve fever and yellow garnet was prescribed for jaundice. If the powder failed to be effective, the apothecary was blamed for having used a substitute, instead of the genuine stone, for the powder prescribed.
Asiatic peoples used garnets as bullets, in the belief that their color would cause them to inflict a more deadly wound. Their use for this purpose has also been mentioned in accounts of our Southwest Indian wars. The garnet is sometimes regarded as a royal gem, due to the preference the Persians have given it as the bearer of their sovereign's image. As an amulet, it was especially favored by travelers, for it was said to protect and preserve honor and health, cure the wearer of all diseases and guard him from all perils during the course of a journey. All of these powers were doubled for the person born in January.
Although the jeweler and his customer alike tend to think of garnet as a single gemstone, there are actually a number of separate species that make up the garnet group. Among them there is a wide range of appearances, based on differences in color, transparency, luster and fire. The various garnets form a group rather than a single species because they differ significantly chemically, even though they are substantially identical structurally. Each species is a silicate of both bivalent and trivalent metals, corresponding to the general formula R3M2(SiO4)3 . In this formula, R is a bivalent metal such as Ca(calcium), Mg(magnesium), Fe (bivalent iron, referred to by chemists as ferrous iron), or Mn (manganese); and M is a trivalent metal such as Al (aluminum), Fe (trivalent, or ferric, iron) or Cr (chromium).
The metals that are in combination in each of the garnet species usually have a profound influence on the color. Although calcium, magnesium and aluminum seem to have little effect on color, chromium, manganese and both valences of iron are powerful colorants in compounds. Each of the species described below is identified by composition. However, it is essential to realize that the structure of all the garnets is the same, and the diameters of the atoms of the several metals combined with silica do not differ materially, so the positions in the lattice filled by a bivalent metal may be filled by some of each of those metals mentioned earlier. Thus, a garnet with a composition representative of the single species listed (i.e., with no or very limited percentages of other atoms) are unknown in nature. Most represent a high degree of replacement of the atoms ascribed to that species by those of one or more of the other species of garnet. For this reason, property variations among the garnets are wide, and the identification of one species from another may be based on an arbitrary line drawn between the two on the basis of property determinations.
Almandite, or Almandine
Almandite (pronounced AL-man-dite), the most commonly encountered species, is the iron-aluminum member of the garnet family and is characterized by its dark, slightly brownish-red to purplish-red color. Chemically pure garnets of any kind are very rarely encountered; they are almost invariably intermediate in composition between species. For example, almandite grades into pyrope, the magnesium-aluminum garnet. Almost all dark-red types contain significant amounts of both magnesium and iron. Most of the garnets in jewelry are either almandite or pyrope. Some gemologists feel that the two may be distinguished by eye, expecting almandite to be more purplish and pyrope a purer red. Almandite that is close to the pure iron-aluminum form is usually a dark, slightly brownish red, but pyrope often has the same color. The name almandite comes from the Latin "alabandicus," the name given by Pliny to the garnets that were found a Alabanda, a town in Asia Minor. In America, it is usually called almandite, rather than almandine.
A distinctive variety of almandite is one that contains enough needlelike inclusions to impart asterism to stones cut in cabochon. This material is usually four rayed, but six rays may be encountered in certain directions.
Pyrope, the magnesium-aluminum garnet, is usually some what more transparent and tends to a slightly purer red (less brownish) than almandite. As a result, there is more likelihood of pyrope being confused with a dark ruby than is true with almandite. In material of average quality, it is virtually impossible to detect any difference between pyrope and almandite. It is only in the finer qualities of pyrope that the higher transparency and slightly purer color appear. Because of the resemblance of fine qualities of both almandite and pyrope to ruby, these garnets have long been described and sold by such misleading names as "Colorado ruby", "Cape ruby", "Arizona ruby" and others. The name pyrope is from the Greek meaning "fiery eyed" or "firelike" in allusion to its deep-red color.
This term is applied to the material that is intermediate in composition between almandite and pyrope and that is distinctive in appearance from either end of the series; i.e., it is violetish or purplish red and more transparent. In addition to the difference in appearance, rhodolite can be distinguished from either parent by the fact that its R.I. is between the two, ranging from 1.75 to 1.78. To those who enjoy a violetish-red color, rhodolite is a lovely stone that resembles closely some fine corundum, from distinctly violetish rubies to the so-called plum sapphire color. It also occurs in a slightly brownish-red color of light tone and a high degree of transparency; in other words, a kind of brownish red that does not resemble that of almandite or pyrope because of a considerably higher degree of transparency and a lighter tone. B.W. Anderson, the British gemologist, has suggested the name "pyrandine" in place of rhodolite. Rhodolite derives its name from two Greek words meaning "rose" and "stone."
This type, which is also called hessonite, essonite and cinnamon stone, is the calcium-aluminum garnet. Grossularite occurs in perhaps a wider range of colors than any of the other garnets. The two most important colors are transparent, rich brownish yellow, resembling fine topaz, and a translucent green, which resembles jade. In recent years, a rich green transparent variety of grossularite has appeared. It bears a close resemblance to demantoid but lacks the strong dispersion of demantoid. In the recent time these Green TP Grossularite was sold in the American market as Emerald, and many jewelers were cheated.
On rare occasions, an opaque, mottled green garnet is seen. This material is a combination of grossularite, hydrogrossular, andradite and uvarovite. Capable of taking a high polish, its R.I. is approximately 1.78 to 1.79 and the S.G. 3.80 to 3.90. The green translucent variety, which resembles jade, often contains tiny black inclusions. The white translucent variety also has a textural resemblance to jade and has sometimes been sold as jade. Rarely, grossularite is also found in a colorless to white form and a rose red. Hessonite, the brownish-yellow variety in transparent form, has a largely unrealized sales potential, because it has a higher luster than topaz and all of the richness of color associated with that gemstone. One of the world's most outstanding hessonites is a 61 1/2-carat cameo carving of the head of Christ in the American Museum of Natural History.
The most important variety of the andradite species is demantoid (deh-MAN-toid). In its finer qualities, it resembles an exceedingly brilliant emerald with strong fire. The name comes from the Dutch "demant" meaning "diamond" so called because of the high luster and dispersion of this garnet. Andradite was named after d'Andrada, a Brazilian mineralogist who made a study of garnet more than a century ago. Demantoid has a deep, rich, slightly yellowish-green color, but its size range is somewhat limited. In ring sizes it is one of the rarest of gemstones. Stones over ten carats are virtually unknown. An unusual antique necklace appeared in Europe in the 1930's that contained five demantoids of more than ten carats each. One weighed in excess of fourteen carats and two were slightly over twelve carats each; all were sold to museums as unusually large stones. In the late 1950's, an 18-carat stone appeared on the New York market. The rich-green color of fine specimens is accounted for by slight replacement of the iron oxide by chromium oxide in the structure. Demantoid has often been called "olivine" and even today many colored-stone dealers continue to use this incorrect name. Since olivine is the mineralogist's name for peridot, the confusion is unfortunate. In addition, olive green hardly describes the color of fine demantoid.
Another variety that is mentioned in most mineralogical texts but seldom encountered by jewelers is the transparent yellow topazolite; however, it occurs in sizes even smaller than the demantoid variety. Melanite is a black variety of andradite that has been used occasionally in place of black onyx or jet for mounting jewelry and other kinds of jewelry in which those materials are used. It has no real significance as a gemstone.
Spessartite, the manganese-aluminum garnet, derives its name from Spessart, Bavaria, where it has been found. The color ranges from yellow-orange to orangey red, orangey brown or reddish brown. In some qualities it resembles hessonite closely, but generally it is more orange to red than that stone. In other words, its color may be considered to be midway between that of the yellow hessonite and the brownish-red rhodolite. Gem quality material, which usually approaches an orange color, is quite rare; however, it is now mined in one or two localities.
There is another garnet species that has the beauty to
be an important gem: the calcium-chromium garnet called uvarovite. Unfortunately,
this lovely emerald green stone is found in such tiny sizes that it would
be suitable for cutting only small calibre. It was named after a Russian
count, S.S. Uvarov, who was president of the St. Petersburg Academy.
One or more of the many garnet species is found in almost every igneous or metamorphic rock type. They may be scattered in submicroscopic grains widely separated within the rock, or they may be a major constituent. Garnet is particularly common in certain kinds of schist in contact zones between various types of rock and later igneous intrusions; e.g., in crystalline limestones and other metamorphosed sediments, and in both volcanic and intrusive igneous rocks. When impure limestones are metamorphosed, either by intrusion of igneous material or by general folding and other metamorphic activity, one of the constituents in the altered rock is almost sure to be grossularite. Micaschist and other metamorphic rocks that are rich in iron and aluminum invariably contain almandite. Andradite is commonly encountered in contact metamorphic zones, particularly those rich in lime, such as altered limestones formed into contact marbles by the action of a basic magma rich in iron. The demantoid variety usually occurs in serpentine. Pyrope is generally associated with basic igneous rocks such as peridotite, dunite and their alteration products. Spessartite is usually found in granites or other rocks that are rich in silica.
The fact that the average layman finds most remarkable is that garnets can grow under certain conditions in solid rock. When the rocks in which they occur are subjected to high temperatures and pressures, the rather dense and compact arrangement of atoms in the garnet is such that the constituents occupy less volume when formed into garnet crystals than in the minerals present before the garnet is formed. As a result, under intense heat and pressure, atoms in solid rock of the necessary composition may rearrange themselves to the patterns associated with certain garnets. Such crystals are usually filled with masses of inclusions made up of constituents that did not fit into the structure of the garnet.
The garnet crystals that occur in schists and other rocks in which they are formed by dynamic metamorphic activities are seldom of gem quality. Although some may be of considerable size, they usually are opaque or nearly so. Those that are not opaque tend to be so badly fractured and full of inclusions that they have little value as gemstones. Garnets found in contact metamorphic deposits are much more likely to be of gem quality. In addition, some of those found in other types of metamorphic deposits in which extreme pressure conditions did not follow crystal growth are more likely to be of gem quality. Almandite crystals several feet in diameter have been encountered; in some deposits, stones of this size are not unusual. A mine in New York State from which garnet is taken to be used as an abrasive contains many crystals over a foot in diameter.
Fine pyrope is found in few places. The small stones that were used so lavishly in jewelry at the turn of the century were mined extensively in the area from which the name Bohemian garnet was derived; i.e., in the northeast section of the old kingdom of Bohemia, near Trebnitz. This quality resembled almandite and lacked the pure red of the more attractive South African stones. Fine pyrope is associated with diamond in pipes in the Union of South Africa. Excellent qualities have come from both the Kimbereley and De Beers pipes. From time to time, very fine qualities in fairly large sizes have come to light; unfortunately, however, the sources have never been divulged and mining plans have failed to materialize. Other sources from which pyrope comes are Southern Rhodesia, Brazil, Australia, Argentina, Ceylon, Mexico and Burma.
Although almandite is the most commonly encountered garnet, only rarely does it occur in gem quality. Gem quality almandite is found in Ceylon, India, Brazil, Alaska and Greenland; star material comes from Idaho. Most gem almandite is found in alluvial deposits as aby-product in the mining of more valuable gemstones.
Rhodolite is found in decomposed igneous rocks and associated gravels in Macon County, North Carolina, in the southwestern corner of the State. It is also found in the gem gravels of Ceylon.
The hessonite variety of grossularite is found chiefly in the alluvial deposits of Ceylon; however, it also occurs in eastern Brazil and else-where. The translucent green variety, which is used to imitate jadeite, is found principally in the Transvaal, South Africa, approximately forty miles west of Pretoria. East Africa produces small quantities of transparent green grossularite.
Unfortunately, the only major sources of fine demantoid are located near the Ural Mountains, on both the Siberian and Russian sides of that range. As a result, high quality material has been virtually unobtainable, except in older jewelry pieces, for a number of years. Evidently, there has been little or no mining since the Soviet forces took over. The major source was the Sisserck district on the Siberian side, where demantoid was mined from serpentine in a tributary of the Chusovaya River. It is also found in other parts of Europe, notably Saxony, Hungary and Italy. Topazolite is almost never used as a gem material, primarily because it is found in sizes too small for practical cutting. It does occur, however, in Gunnison County, Colorado; Germany; Norway; New jersey; and Russia.
Gem quality spessartite is found in the gem gravels of Ceylon; in Minas Geraes, Brazil; and in Madagascar. It has also been mined in Nevada and in San Diego County, California. A significant amount of gem quality material has been available from the latter source in recent years.
Uvarovite was discovered in the Ural Mountains, but has also been found in other mountainous, areas, notably in the Pyrenees and the Himalayas.
The transparent garnets may be cut in any style, including brilliant, step and mixed. The cabochon form is often used for pyrope and almandite and for the green and other translucent varieties of grossularite. The hollowed cabochon style is sometimes favored for very dark specimens of almandite and pyrope, Although the emerald cut is sometimes used for demantoid, its high dispersion is more effectively displayed by the round brilliant cut.
Since garnet may fracture with changes in temperature, this point should be kept in mind during the fashioning operation; otherwise, only the usual care is necessary. Although some lapidaries state that wet sanding is beneficial, tests have shown that dry sanding is satisfactory. Cabochon material polishes most effectively on a felt buff with Linde A as the polishing agent; the same agent on a tin lap is suitable for faceted stones. Crown angles of 37° and pavilion angles of 42° are recommended. Only asteriated material requires orientation, and this can be accomplished by the usual method of wetting the rough ground convex surface to locate the apex of the star.
|Chemical Composition||The composition of the garnets may be expressed by the formula R3M2(SiO4)3, in which R stands for any of the elements calcium, magnesium, ferrous iron or manganese, and M represents any of the elements aluminum, ferric iron or chromium.|
|Crystallographic Character||Cubic system. Habit: rhombic dodecahedron, trapezohedron or a combination of the two forms.|
|Hardness||Almandite, 7 1/2; andradite, 6 1/2 to 7. All others, 7 to 7 1/2|
|Toughness||Fair to good.|
|Cleavage||Very difficult. May show indistinct cleavage along planes of the dodecahedron.|
|Specific Gravity||Spessaritite: 4.12 to 4.18; normal, 4.15
Almandite: 3.93 to 4.17; normal, 4.05.
Andradite: 3.81 to 3.37; normal, 3.84.
Rhodolite: 3.74 to 3.94; normal, 3.84.
Pyrope: 3.62 to 3.87; normal, 3.78.
Grossularite (hessonite): 3.57 to 3.73; normal, 3.61.
Grossularite (green, translucent): 3.45 to 3.50; normal, 3.47.
Grossularite (green transparent): 3.59 to 3.63.
|Characteristic Inclusions||Almandite: needlelike crystals, usually coarse
(1) zircon crystals with halos caused by radioactivity
(2) and irregular, rounded inclusions of low relief.
Hessonite: stubby, rounded prisms and a "heat-wave" effect
(3) Green grossularite: numerous tiny black inclusions.
Demantoid: curved, radiating, needlelike ("horsetail") inclusions of a yellowish-brown hair like form of asbestos
(4) Spessartite: wavy, irregular, featherlike liquid inclusions
(5) Pyrope: needlelike crystals and irregular, rounded inclusions similar to almandite.
|Degree of Transparency||Transparent to opaque.|
|Luster||Polished surfaces are vitreous to subadamantine; fracture surfaces are greasy to vitreous.|
|Refractive Index||Andradite: 1.856 to 1.895; usually, 1.875.
Spessartite: 1.79 to 1.82; usually, 1.81.
Almandite: 1.77 to 1.810; usually, 1.79.
Rhodolite: 1.74 to 1.77; usually, 1.76.
Pyrope: 1.72 to 1.75; usually, 1.746.
Grossularite (hessonite): 1.74 to 1.75.
Grossularite (green, translucent): 1.72 to 1.73.
Grossularite (green transparent) 1.736 to 1.742
Grossularite: . .028
Pyrope and Spessartite: .027
Rhodolite: 026 Almandite: 024
|Phenomena||Asterism in almandite|
|X-Ray, Fluorescence||All are inert except green grossularite, which exhibits an orange fluorescence.|
|Color-Filler Reaction||Demantoid shows a reddish color; all other red garnets are dark red.|
|Absorption Spectra||Pyrope: a broad band from
about 5200 to 6200 A.U. plus one fairly broad but faint line
just above 5000; Fine-quality material may show chromium lines
in the red. Almandite: Usually, three broad,
strong lines at about 5700 to 5800, 5200 to 5300, and 5000 to
5100 A.U.; in addition, it has faint lines at 4760, 4620, 4380
and 4270. Rhodolite: same as almandite.
Grossularite: no diagnostic spectrum.
Andradite (demantoid): usually, some sharp lines in the red at about 6900 and 7000 A .0 , and an intense line at approximately 4400 A.U.
Spessartite: very weak lines in the blue at 5050, 4880 and 4610 A.U.; also, a strong but narrow band at 4300 in the violet.
Uvarovite: no diagnostic spectrum.
Effects Caused by:
|Heat||Most garnets fuse easily before the jeweler's torch or the blowpipe; only uvarovite is almost infusible. All species lose transparency when heated, but regain it on cooling. Subject to fracture with abrupt temperature changes.|
|Acids||Andradite may be affected slightly with prolonged immersion in concentrated acid. The other species are unaffected.|
|Irradiation||No data available.|
The identification of pyrope poses more problems in theory than it does in actual practice. This is because its properties overlap those of spinel, which occurs in a variety that is almost identical in appearance to pyrope. Theoretically, a pure magnesium-aluminum pyrope has a refractive index of 1.705. In nature such a material is nonexistent, for all practical purposes, and the lowest R.I. likely to be encountered is 1.73. Spinel, on the other hand, usually shows a reading just below 1.72. On occasion, pyrope with refractive indices below 1.73 has been reported, but this is very rare. Only once has the GIA laboratory encountered a pyrope in this range. This stone, which was from a deposit discovered by a pair of prospectors in the Southwest, had an index just above 1.72. To our knowledge, the deposit has never been worked. Stones from Ceylon are sometimes as low as 1.73, but more often are 1.74 to 1.75. South African stones are nearer the higher figure. The dividing line between pyrope and almandite is arbitrary, but the usual upper limit for what is considered as pyrope is 1.75.
The inclusions of pyrope and spinel are helpful in distinguishing between these two stones, for spinel often contains well-formed octahedral, whereas the inclusions in pyrope are more likely to be irregular, rounded grains of low relief and occasionally needlelike inclusions.
By the use of the spectroscope, it is also possible to distinguish between pyrope and spinel of a comparable color. In general, the absorption in a pyrope is considerably greater than that of similarly colored spinel. Spinel shows a broad but rather weak absorption band from approximately 4900 to 5950 A.U., whereas pyrope shows a broad band from about 5200 to 6200 A.U., plus one fairly broad line just above 5000 A.U.; however, it is rather faint. The finest pyrope from Arizona may show chromium lines in the red.
Pyrope can be distinguished readily from garnet-and-glass doublets by examination under magnification or by immersion. The R.I. of garnet crowns on doublets is almost always in the almandite range, rather than in the pyrope range. The R.I. of glass used for gem purposes rarely exceeds 1.69; however, if it is in the range expected of pyrope, the specific gravity is much higher than that of pyrope.
The resemblance of fine pyrope to dark ruby should cause no difficulty in identification, because of the difference in R.I. between the two stones. However, it is a simple matter to effect a separation by the presence of strong dichroism in ruby and the total lack of it in pyrope. Ordinarily, the polariscope will suffice to distinguish between these two stones without difficulty. When the polarizers are in the parallel position (i.e., the position that produces maximum light transmission) dichroism should be evident in a ruby, but it will never be present in a garnet. Other gemstones that resemble pyrope can be distinguished therefrom readily by R.I. and/or S.G.
Almandite, too, is sometimes confused with rubies of darker color, as well as with doublets, glass and synthetic ruby. It can be distinguished readily from ruby or synthetic ruby by its lack of dichroism. In addition, both synthetic and natural ruby may fluoresce rather strongly in a bright orange-red color under long-wave ultraviolet light, whereas almandite fails to fluoresce. This test, however, is not recommended, since fluorescence varies in rubies of different depths of color and from different sources. Because the dichroism is so strong that it is readily evident in the polariscope, this should suffice. Tests of this kind are usually much more dependable than ultraviolet fluorescence.
The absorption spectra of ruby and synthetic ruby are identical, and differ markedly from that of almandite. Almandite usually shows three broad, strong lines at about 5700 a to 5800 A.U., 5200 to 5300 A.U., and 5000 to 5100 A.U.; in addition, it has faint lines at 4760, 4620, 4380 and 4270. Ruby has three sharp, strong lines in the blue; broad absorption from 5000 to 6000 A.U.; and very sharp, strong lines between 6500 and 7000 A.U. in the red. Each has such a distinctive spectrum that it is a certain means of separation.
Almandite can be distinguished from almandite-topped doublets by magnification. Doublets other than red can be distinguished easily by immersing them and examining them parallel to the girdle. No glass used for gem purposes has a refractive index approaching that of almandite, which is in the 1.79-1.80 range.
The lovely rhodolite is duplicated best in appearance by corundum that is on the borderline between ruby and so called plum sapphire; by fine red tourmaline and spinel; and, to a degree, by doublets and glass. An occasional zircon may also have an appearance somewhat akin to that of rhodolite. The polariscope and dichroscope tests described earlier suffice to distinguish between rhodolite and either natural or synthetic corundum. Refractive-index readings distinguish most of the other imitations, with the exception of garnet-topped doublets. (Rhodolite may have an index anywhere between 1.74 and 1.77.) In this case, magnification or a luster difference between portions of the crown or between crown and pavilion should serve the purpose. The strong doubling of zircon is sufficient to distinguish it from rhodolite readily under magnification.
Grossularite differs from the other members of the garnet family in that its two principal varieties are rather widely separated in properties. The foremost gem type of grossularite is the transparent hessonite, which bears a fairly close resemblance to fine topaz. Hessonite is usually said to have a granular appearance; i.e., when examined under magnification, it reminds one of a very thick sugar solution. Anything viewed through such a solution appears as though seen through heat waves coming from a hot surface In the summer sun. In addition, stubby, rounded, prismatic Inclusions of low relief are common under magnification.
Hessonite has a refractive index of approximately 1.74 to 1.75 and a specific gravity that varies from approximately 3.57 to 3.73. The other major variety of grossularite is the translucent green material resembling jade; it has an R.I. of about 1.72, or slightly higher, and an S.G. of 3.45 to 3.50. Transparent grossularite has an R.I. of 1.736 to 1.742 and an S.G. of approximately 3.59 to 3.63.
Distinguishing hessonite and some of the materials that have a very similar appearance should not be difficult. Topaz is much lower in R.I., although the S.G.'s of the two are fairly close. The orange to flame colored zircon is easily separated by its obvious birefringence that is visible under magnification. In addition, zircon has much higher properties. It would be almost unheard of to confuse diamond and hessonite, if both stones were examined under magnification, for the granular heat-wave effect of grossularite is distinctive and nearly always obvious under magnification. The fact that the R.I. of diamond is above the limits of the refractometer, so that a dark shadow extends all the way to 1.81, and that hessonite has a distinct reading in the 1.74 to 1.75 area also suffices to distinguish the two readily.
Almandite-and-glass doublets also imitate topaz and hessonite. As outlined earlier, magnification and/or immersion permit separation between the imitation and the natural stone. Other stones that occur in a yellow to brown color, such as tourmaline and beryl, are easily distinguished by eye alone. Glass could be made in this color and with this R.I., but it would have a much higher S.G. at such a high level of refractivity. It would also be very soft.
The green translucent variety of grossularite is confused most often with jadeite, nephrite, aventurine quartz, serpentine and perhaps idocrase. All of the gemstones that this jade like garnet resembles are doubly refractive, whereas garnet is singly refractive. Although this may prove of value in identification, the grossularite usually transmits a certain amount of light between crossed Polaroid. Unless the top Polaroid is turned to determine whether more light now passes, it is easy to make a mistake in this identification. The polariscope reaction is sometimes very difficult to distinguish from double refraction; however, it is worth checking, for it does give a singly refractive reaction, the other materials mentioned above are eliminated immediately.
Even carved objects are usually well enough polished in places so that the spot method of refractive index determination may be used to advantage. Since jadeite is 1.66, nephrite 1.61, serpentine 1.57 or lower, and idocrase about 1.71, a careful reading will suffice to distinguish the roughly 1.725 reading of grossularite from those of the other gemstones.
Grossularite almost invariably contains small black inclusions. Such inclusions may also be seen sometimes in some of the other gemstones, but they are very rare in jadeite. Glass imitations of jade that may resemble grossularite are invariably lower in refractive index and are easily detected on this basis. In addition, glass is almost certain to contain numerous bubbles just beneath the surface; these bubbles are visible despite the fact that the material is translucent rather than transparent. The only other stone in this group that has an S.G. even close to grossularite is idocrase, but it is only about 3.30, considerably below the 3.45 to 3.50 figure expected in grossularite. Almost any of the other rarely encountered jade substitutes, such as fluorite, serpentine and translucent emerald, also are distinguishable on the basis of their lower physical properties.
The only variety of andradite of any significance in gem testing is demantoid, but it is distinguished easily from most other green gems by its high luster and a degree of dispersion that exceeds that of diamond. The stone with which it is most frequently confused in testing is green zircon, which usually has an R.I. on the order of 1.81 to 1.815; this is just above the limits of the refractometer. Rarely, however, a reading may be seen for green zircon. Although it reacts as a doubly refractive material in the polariscope, it rarely shows visible doubling of back facets, which is a characteristic of all other zircons.
However, usually it is possible to obtain an interference figure in green zircon without too much difficulty, which is not possible with singly refractive demantoid. Demantoid usually shows a certain amount of anomalous double refraction, which may cause confusion in testing.
The key to easy identification of demantoid is the almost invariable presence of yellowish-brown asbestos fibers that usually curve outward from a center within the stone. These are often in bundles, which had led to the descriptive term "horsetail" inclusions. In other words, a bushy "tail" is formed by the fibers, which join at a single point and spread out slightly in a curved path. On occasion, they appear to radiate through a full circle from a common point. No comparable inclusions are likely to be encountered in green zircon or any other green gem.
Under the spectroscope, demantoid usually shows some sharp lines in the red (caused by chromium) at about 6900 and 7000 A.U., and an intense line at approximately 4400 A.U. Green zircon usually shows some bands often many the most important of which is at about 6530 A.U. There may or may not be other bands in the spectrum, such as a narrow one at 5200 A.U. Although the spectroscope may be of some use in this identification, inclusions and the polariscope are usually sufficient to identify demantoid readily and to distinguish it from green zircon.
Demantoid is very readily distinguished by R.I. from all other green stones that may resemble it, with the possible exception of green diamond or a garnet topped doublet when the garnet top has a 1.81 R.I. The doublet is easily distinguished by either immersion or by careful inspection under magnification. Inclusions, cleavage, the nature of the girdle surface (not usually polished on diamond) or S.G. separate diamond and demantoid. There should be no difficulty in distinguishing demantoid from peridot, emerald or green tourmaline. The rare and beautiful green sphene, which also has indices above the limits of the refractometer, is easily separated by its high birefringence, as evidenced by the very strong doubling of facet junctions under magnification.
This species is rather rarely encountered as a gemstone. It is most likely to be confused with zircon or other garnets. In general, it does not have the odd internal appearance that is associated with hessonite, nor does it show the birefringence of zircon in the form of doubling of facet edges under magnification. Spessartite has a usual R.I. of 1.81, but it is not at all uncommon to encounter material with an index within the range of the refractometer to perhaps 1.79 or 1.78. It can be separated from other garnets on the basis of R.I., S.G. and color of the stones that have a comparable color, none approach it in physical properties.
Brownish-red to reddish-brown garnets are very inexpensive. The small rose-cut stones used in so-called Bohemian type jewelry are only worth the cost of cutting; therefore, they are usually sold by the ounce, rather than by the carat. Violet and purple stones, on the other hand, are becoming increasingly scarce in large sizes and fine quality, thus making it difficult to match pairs of stones. Red and slightly purplish-red stones are not too difficult to obtain, but in large sizes they are fairly expensive.
The value of demantoid increases as it approaches the color of fine emerald. Parcels of greenish-yellow to yellowish-green stones ranging from a few points to about one-half carat in size are often available for $4 to $80 per carat. Because of the high dispersion of demantoid, they are especially suitable for pave work around one of the opaque or translucent gemstones.
Hessonite never attains the value that its rarity would suggest. However, a well-cut specimen of fine quality has an appearance that is seldom duplicated in other gems. If such a stone is attractively mounted, it is sure to invite comment, questions and sales.
Green grossularite is most often encountered as a jade substitute, yet it has merits of its own. When considering this gem for stock, it is wise to choose only the finer translucent stones, since the opaque qualities are not particularly saleable. The difference in price is slight, but the difference in appearance is great.