Lets Talk Gemstones

By Edna B. Anthony, Gemologist
(Contact the author for permission to reproduce this article in any form.)
P.O.# 62653; COLORADO SPRINGS, CO. 80962

Let’s Talk Gemology
By Edna B. Anthony, Gemologist

Garnet Group

[A NESOSILICATE]
The Ugrandites

Uvarovite

It was noted in the Let’s Talk Gemstones article introducing garnets that garnets containing calcium include the species uvarovite, grossular, and andradite. These are frequently referred to as the Ugrandites. Uvarovite [calcium chromium silicate: Ca3Cr2(SiO4)3] develops in the cubic crystal system and forms a series with grossular. It was named to honor the Russian Count S. S. Uvarov, once a president of the St. Petersburg Academy. The chrome-bearing emerald green mineral occurs as small, opaque crystals that rarely yield a transparent portion suitable for cutting a tiny faceted gem. Seams in chromite deposits permit the deposition of usually well-formed rhomboid dodecahedral crystals with striated faces. The icositetrahedron (isometric trapezohedron) habit and combinations of the two habits are known.

The largest and finest uvarovite crystals (1 to 2 inches) are found in an atypical deposit in a copper mine in Outokumpu, Finland. Deposits in the Sverdlovsk region near Saranaya and Sysert in the Urals are also well-known. Other sources include Quebec, Canada, the Texas mine in Pennsylvania, northern California, Oregon, Spain, Italy, Polish Silesia near Jordanow, at Makri in Turkey, and India.

Uvarovite develops in a metamorphic environment in serpentines with chromite (ferromagnetic formations in South Africa, Norway, and Finland are examples) and in metamorphosed limestones. It is highly resistant to attack by acids until fused. Fusion occurs with great difficulty. Crystals leave a white residue on a streak plate and exhibit a sub-conchoidal to hackle fracture surface. Its hardness, 7.5, approaches that of topaz, and the density can vary from 3.4 to 3.8 on the Mohs scale. The usual reading falls between 3.71 and 3.77. Although brittle and fragile, there is no discernible plane of cleavage. The luster can vary from sub-adamantine to vitreous to resinous. The single refractive index reading is 1.86 or 1.87. In his Gemstones of the World, Walter Schumann lists the diaphaneity of uvarovite as “transparent to translucent”, but he also states there is no dispersion. Other references make no mention of this property, nor of any inclusions typical of this specie.

Uvarovite’s gemstone characteristics are outstanding, but its rarity and the very small size of its crystals make it practically unknown as a faceted gem. Fine crystals closely resemble the best emerald. Today, the crusts of small crystals on matrix are available at many trade shows, and the public is more aware of this lovely mineral. Many modern designers are using the druzy material for unusual freeform jewelry creations.


Andradite

The andradite species of the calcium-containing garnets was named to honor the mineralogist Jose Bonifacio de Andrada e Silva. The brown and green varieties of andradite often occur in regional contact zones of metamorphosed impure siliceous limestones. [Tsavorite, the rich green garnet colored by vanadium and chrome, is a grossular garnet.] Brown material is seldom faceted except for collectors. Schists and serpentines yield the demantoid and topazolite varieties of andradite. The melanite and schorlomite varieties require an alkali-rich environment of igneous rocks.

Andradite [calcium iron silicate] and grossular [calcium aluminum silicate] garnet are the end members of a series. The difference in the chemical compositions of the members of the series and variations of temperature and pressure of the environment where crystallization takes place can produce the unusual iridescent garnet. Iridescence occurs with the incorporation of extremely thin layers of grossular in andradite’s isometric structure. The slight differences in the refractive indices of the layers of the two minerals affect the light reflected from within the stone to the observer and cause the spectral effect. Rare chatoyant material, caused by inclusions of fibrous amphibole (byssolite), is found in San Benito County in California. Fine green, chrome-bearing crystals occur in a deposit in Korea.

The specific gravity of andradite can vary from 3.7 to 4.1. The iron content negates the presence of luminescence, and fusion produces a black magnetic globule. Fracture is uneven to conchoidal. The diaphaneity can vary from transparent to opaque with a luster range from vitreous to metallic. Large gem quality crystals are unusual, so faceted gems are typically rather small. With a normal refractive index range of 1.88 to 1.94, andradite has the highest dispersion (0.057) of all the garnets. However, the tremendous fire of these gemstones is frequently masked by the body color. Despite garnet’s tendency to brittleness, a hardness of 6.5 to 7, and the absence of cleavage planes combine with andradite’s extreme dispersion to make it suitable for use in all types of jewelry.

Demantoid is the most valuable of the andradite garnets. The finest material is found in metamorphic formations and alluvial gold-bearing deposits in the Ural Mountains in Russia, but other sources are located in Italy, Kenya, Tanzania, and Zaire. Demantoid has been called Ural emerald and Siberian chrysolite. Production today is very limited, so antique jewelry is a source of this prized gem. Demantoid’s color can vary from a very pale green to a rich medium green colored by chrome. Its density varies from 3.82 to 3.88. A singly refractive index of 1.88 to 1.89 is the norm. The presence of chrome creates the “red” reaction in the Chelsea filter and, at times, the chrome spectrum is visible. Faceted gems of over a carat are extremely rare. The collections of several museums in the former USSR contain many of the world’s finest and largest demantoids. The Smithsonian cabinet in the United States includes one demantoid of 10.4 carats, as well as others of 4.1 carats, 3.4 carats, and 2.3 carats. “Horsetail” inclusions, composed of fine hairlike fibers of byssolite, are common and exclusive to demantoid, and these are diagnostic when found. Such inclusions weaken the facets and edges and increase a gemstone’s fragility. Round brilliant and mixed cuts best exhibit demantoid’s gem properties. Square or rectangular step cuts are rare. Careful examination of its physical and optical properties is necessary to distinguish demantoid from some green zircons with a weak birefringence, as well as from green grossular and green YAG.

 Topazolite garnets rival demantoid in gemstone qualities, but the exceptionally small, pale to dark yellow and yellowish-green crystals, found in metamorphic deposits in the Italian and Swiss Alps, seldom yield gems of a practical size for use in jewelry. Faceted gems larger than 2 to 3 carats are very rare. Topazolite crystals (density of 3.77 to 3.81), associated with the unusual andradite cats-eye material from San Benito County in California, exhibit a somewhat lower refractive index (1.855 to 1.877) than the usual 1.887 of the European material. It is known that an 18-carat gem was purchased in New York by a private collector. An extremely large green topazolite crystal, weighing approximately one ounce, graces the cabinet of a collector in California. In his Color Encyclopedia of Gemstones, Dr. Joel Arem tells us that this one ounce specimen would yield faceted gems of over 20 carats.

Melanite occurs in igneous formations on the island of Elba and in Trentino, Vesuvius, and Monte Somma, Italy, as well as in Kaiserstuhl, Germany, France, Norway, and Magnet Cove, Arkansas, and Colorado in the United States. Titanium oxide can make up as much as 5% of the chemical composition of this opaque material. The name is derived from the Greek melas, meaning black, but a dark red is also known. Melanite exhibits an approximate specific gravity of 3.9 and a refractive index of 1.89. Its vitreous to metallic luster made faceted melanite gems popular for use in mourning jewelry during the Victorian era.
Schorlomite is also a titanium rich andradite found in volcanic alkali-rich formations. It is not used as gem material. Brown andradite is seldom faceted and used in jewelry.


Grossular

A greater variety of colors occurs in grossular garnet [calcium aluminum silicate – Ca3Al2(SiO4)3] than in any of the other garnet species. Grossular forms series with andradite and uvarovite. Its name is derived from the botanical name of the gooseberry (Ribes grossularia) that it resembles in form and color. The diaphaneity ranges from transparent to opaque. Under the microscope, transparent grossular exhibits a distinctive characteristic described by Richard T. Liddicoat in his Handbook of Gem Identification as “it looks like a saturated sugar solution with light passing through it.” In the Color Encyclopedia of Gemstones, Joel Arem attributes this swirled effect to “included diopside crystals and irregular streaks at grain boundaries.” Pure grossular is colorless and rare. The presence of small amounts of trace elements, iron, manganese, chrome, or vanadium, produces its various shades of yellow, brown, green, black, orange-brown, red, pink, gray, and white. [It is interesting to note that a purple-red grossularite is mentioned by Liddicoat that can be “almost semi-transparent.” It “usually transmits much more light than rhodolite”; and “is recognized by a refractive index between 1.70 and 1.73 and a specific gravity from approximately 3.25 to 3.5.”] A content of more than 2% iron produces the deeper tones of yellows, browns, and greens. Typical environments for this common mineral are regional and contact-metamorphosed zones of impure calcareous rocks (calcium carbonate rocks, i.e. limestones, shales, and mica schists). Less frequently, serpentines and metamorphosed basaltic lavas are sources of the isometric dodecahedral or trapezohedral crystals. A refractive index of 1.74 is the norm for grossular. Its luster can vary from vitreous to resinous. The lighter colors do not mask grossular’s high dispersion (0.027), and well cut stones make exceptional gems. Like other garnets, grossular exhibits no cleavage, but brittleness is a characteristic. Its hardness (6.5 to 7.0+) and the density (3.4 to 3.71) are variable. Joel Arem’s Color Encyclopedia of Gemstones states that grossular seldom exhibits luminescence in ultraviolet light, but exposure to x-rays reveals an orange glow in all massive material and in many faceted gemstones. In A Guide in Color to Precious and Semiprecious Stones, Jaroslav Bauer and Vladimir Bouska state that, “In contrast to other garnets, grossular strongly luminesces in ultraviolet light.” Since the refractive indices of grossular and synthetic spinel are similar and both frequently house rod and acicular inclusions, Dr. Liddicoat advises that grossular’s “weak green fluorescence to shortwave” and “weak orange to longwave” may be helpful in identification. Other inclusions common to the varieties of grossular include actinolite, short, stubby low relief crystals of diopside, and crystals of zircon and apatite. Uncommon variations of the physical and optical properties of the grossular varieties from the normal characteristics of the garnet group are presented in the paragraphs devoted to the various varieties.

Massive grossulars

The important sources of the massive varieties of grossular are located in South Africa, China, and Myranmar (Burma). Green chrome-bearing masses (R.I.=1.738 to 1.742 and S.G.=3.63) are found in Pakistan. Similar material found in Tanzania exhibits a density of 3.68 with a refractive index range of 1.742 to 1.744.

Hydrogrossular

Hydrogrossular [Ca3Al2(SiO4)3(OH)4], a crystalline component of massive grossulars, occurs in numerous locations and several colors. Quite large cabochons can be fashioned from this translucent material. New Zealand hydrogrossular is known as rodingite. The Transvaal region in South Africa produces manganese-bearing pink material (R.I.=1.675 to 1.705 and S.G.=3.27), gray containing zoisite, and green material, resembling jade, which is often called Transvaal or South African jade. The presence of visible small black inclusions in this cryptocrystalline material can help to distinguish high quality “Transvaal jade” from jadeite. A waxy luster and a splintery fracture can be characteristics of the compact homogenous material.

 Leuco [white] garnet

The rare crystalline grossular called leuco or white garnet found in Italy (Cantanzaro), Norway (Telemarken), and at Jordansmuhl, Poland is seldom faceted and used as a gemstone. No other gem mineral of its color range exhibits a singly refractive index of 1.725. Colorless crystals with a refractive index of 1.733 are found at the Jeffrey Mine near Asbestos, Quebec, Canada. A source near Georgetown, California produces material with a refractive index of 1.737 and a density of 3.506. White crystals are frequently associated with the greenish and pink material from Lake Jaco, Chihuahua, Mexico. Myanmar and China produce white garnet that is frequently carved by Chinese artists.

Rosolite [xalostocite, landerite]

Marble deposits in Lake Jaco, Chihuahua, Mexico yield pink grossular known as rosolite. The manganese-bearing commonly opaque concentrically zoned crystals can have a diameter of up to 5 inches. Material from this locale has been known and used for years by the inhabitants of the southwestern part of the U.S. Lesser known is the sometimes-transparent rose-pink material from Xalostoc, Morelos, Mexico, where large well-developed crystals associated with vesuvianite are extracted from the granular limestone formations. Well-cut stones from these crystals are beautiful gems. Fine pinkish grossular crystals are found at the Jeffrey Mine in Quebec, Canada.

Green grossular

The incorporation of iron into its chemical composition accounts for the colors of most of the green grossular garnets. Greater amounts of the element create the dull turbid tones often associated with this material. Concentrations of less than 2% iron permit the development of very attractive lighter hues. Although singly refractive with a luster that is not as strong, it is frequently confused with green zircon and green tourmaline. Gem quality crystals are found in the gem gravels of Sri Lanka, Tanzania, and Pakistan.

Tsavorite [tsavolite]

Since its discovery several decades ago in southern Africa near the Tsavo National Park, Kenya, this bright green grossular has rapidly gained favor with the public. Original analysis disclosed traces of chrome to which its intense color was attributed. More recent examinations have determined that, although chrome is present, vanadium is responsible for its brilliant hue. The rare crystals seldom exceed a size of more than a carat. In addition to the original source from the Lualenyi, Kenya mine, deposits have been found in Tanzania and Pakistan. Tsavorite exhibits a refractive index of 1.743 and a density of 3.61. According to Joel Arem, exposure to ultraviolet light elicits no reaction. In his Handbook of Gem Identification, Richard T. Liddicoat, Jr. states that, “The transparent green material may show a weak-to-moderate red fluorescence under both short- and longwave ultraviolet light.” [Upon exposure to shortwave ultraviolet light, only one of several tsavorite gems made available to the author from private sources exhibited a weak dull red fluorescence.] Examination under the microscope often reveals tiny white crystals in healed fractures. Acicular actinolite crystals and mirror-like plates of hematite are seen less frequently.

Hessonite [essonite, cinnamon stone, yellow garnet]

An extensive essay on this transparent to translucent variety of grossular garnet appears in Precious Stones, Volume 2, by Max Bauer. Several of his observations were found nowhere else. Collectors of antique jewelry may be surprised to learn that as late as 1904 hessonite gems were “usually mounted upon a burnished foil, rarely set `a jour”. It was not until the close of the eighteenth century that this gem was identified as a garnet. Until then, it was believed to be zircon (hyacinth, jacinth). The gem gravels of Sri Lanka, especially those in the Matura district, are major sources of hessonite and zircon. The water-worn pebbles found there, some weighing as much as five pounds, yield the finest hessonite gem material. Hessonite’s resemblance to zircon is uncanny and the confusion of the gems and the terms persists even today. [Such confusion with identification would seem more likely when confronted with spinel of similar color.] Both iron and manganese are constituents of hessonite. Differences in the amounts account for the variations in color from a rich reddish brown to orange to pinkish-orange to pale honey-yellow. Bauer states, “The color varies somewhat according to the distance at which the stone is held from the eye. It appears distinctly red only when held some distance away; close to the eye it often appears nearly pure yellow. Hessonite is also remarkable in that its color by lamp-light is considerably more brilliant and fiery than by day.” Bauer also tells of gem quality pea-sized crystals of hessonite, each usually enclosing a grain of quartz, that are found in the Alp-Lolen near Grisons, Switzerland. He mentions that a locale in the Ala valley in Piedmont is the source of hessonite druses found in crevices of the serpentine formations. Dark green chlorite and pale green diopside are frequently associated with these druses. Brazil, Greece, and the Harts Range in Australia’s Northern Territory produce gem quality hessonite.

The Greek word esson, meaning inferior or less, is the root for hessonite’s name. In spite of the fiery brilliance of well-cut stones, yellow-brown gems were regarded as the least valuable of this material. At times, hessonite exhibits a highly vitreous luster that can incline to resinous, and an anomalous double refraction is known. Because its refractive index (1.74) is so close to that of methylene iodide, it becomes almost invisible when immersed in the fluid. The diagnostic “treacly” appearance of the interior of the crystals has been described as: “scotch in water”, “heat waves over hot pavement”, “roiled”, “sugar candy”, “syrupy”, “swirled”, and “granular.” The specific gravity of hessonite ranges from 3.6 to 3.7, and it fuses easily to a greenish glass. Though the iron content is minimal, a slight reaction to a magnet can be present.

The large size of some crystals and a hardness of 6.0 permit the economic use of this gemstone for carvings, engraved cameos, and intaglios. The collection in the American Museum of Natural History includes a 61.5 carat cameo head of Christ.