Photo of Grossular Garnet in Processed & Rough Form
Grossular Garnet is a mineral with a hardness of 7 out of 10 on the Mohs scale of mineral hardness [?]. These Cubicly structured gems are made of calcium aluminum silicate, their full chemical compound being Ca3Al2(SiO4)3.
Grossular is a nesosilicate in the Garnet group, with an isometric crystal system. Together with uvarovite and andradite, the three are known as the "ugrandite garnets," an isomorphous series of calcium garnet minerals.
It appears with dodecahedral or trapezohedral crystals of various colors; normally characterized by a green color, but also may be colorless, pale green or milky when pure, cinnamon-brown to orange when containing iron (hessonite/essonite variety/cinnamon stone/hyacinth/jacinth) and magnificent emerald-green with chromium (known commercially as tsavorite/tsavolite/tsavolithe from East Africa). The green color is known as grossularite (green gooseberry) or gooseberry stone. Massive variety of grossularite is named as hydrogrossularite.
The golden-yellow is called cinnamon stone.
The pink or purple colors are known as landerite, rosolite, and xalostocite.
Amber-colored varieties of grossular are called Succinites.
The reddish-brown variety is mistakenly called hyacinth. The reddish-orange variety is erroneously named jacinth.
The massive pale green variety of grossular from Transvaal, South Africa is known incorrectly as Transvaal jade or African jade. Some grossular show oil streaks and sometimes are called treacle.
Grossular is very hard, heavy, fragile, has no cleavage. It is transparent with vitreous luster, infusible, and insoluble. It is fluorescent in ultraviolet light, and when exposed to x-rays shows faint greenish-yellow fluorescence.
It is a mineral typical of regional and contact metamorphism of calcareous rocks, associated with calcite, wollastonite, vesuvianite, diopside, and scapolite.
Superb, clear hessonite crystals are found in the gem-bearing sands of Sri Lanka, in the rodingite bodies of California (Ramona), the western Alps (Val d'Ala, Val d'Aosta and Valle del Sangone, in Val della Gava, Liguria–all in Italy), at Asbestos (Canada), and Maharitra (Madagascar). In the USA beautiful crystals are found at Minot (Maine), Warren (New Hampshire), and Eden Mills (Vermont), and the mineral is common in several places in California and Colorado.
Grossular is singly refractive, and is distinguished from green tourmaline and green zircons, which are both birefringent.
Other names for grossular include:
Californite; a compact, white variety of grossular or hydrogrossularite garnet used as an ornamental stone from Fresno, Siskiyo, and Tulare Counties, California, USA.
Piedra de grosella; Spanish term for grossularite garnet.
Romanzovite; a brown to dark brown variety of grossularite garnet from Finland.
Rose garnet; a large crystal of pink or rose pink variety of grossularite garnet, which occurs in marble found in Mexico. Also called landerite, roselite, and xalostocite.
Roselite; a large crystal of pink or rose pink variety of grossular garnet, which occurs in a creamy marble. Found at Xalostoc, Mexico. Also called landerite and rose garnet. Locally the stone is known as xalostocite. Also spelled rosolite.
Vanadium garnet; a variety of grossular garnet, which sometimes contains up to 4.50% vanadium oxide.
White garnet; a translucent variety of grossular garnet resembling white jade, found in Tanzania, Africa. Used as a gemstone.
Wiluite; a greenish variety of grossular garnet from Wilui River, Siberia, Russia.
Grossular is often mistakenly called the following:
African jade; a misleading term for a green, compact variety of grossular garnet from Africa. Also called Transvaal jade or South African jade.
Oregon jade; a misleading term for massive grossular garnet from Oregon, USA.
South African jade; a misleading term for massive grossular garnet from Transvaal, associated with black chromite. Used as decorative items. Also called Transvaal jade, African jade.
White jade; a misleading term for a translucent variety
of grossular garnet found in California, USA.
It is often confused with emerald and demantoid.
Why does Grossular show such diverse colors?
Grossular's chemical structure allows for various trace elements to create different colors: chromium produces green (Tsavorite), iron creates yellow-green (Mali Garnet), and manganese can yield pink hues. Pure grossular is actually colorless.
What makes Tsavorite different from Emerald?
While both show rich green colors, Tsavorite (green grossular) has higher brilliance, is untreated, and typically shows fewer inclusions than emerald. It's also considerably rarer but often less expensive.
Why is Mali Garnet unique?
Mali garnets from West Africa show distinctive mint-green to yellow-green colors unseen in other garnets. Their unique coloration comes from specific combinations of iron and other trace elements during formation.
What causes color-change in some Grossulars?
Rare color-change grossulars, mainly from Tanzania and Madagascar, show green in daylight and pink/red in incandescent light. This phenomenon results from specific trace element combinations affecting light absorption.
How is Hessonite related to Grossular?
Hessonite is the cinnamon-brown to orange variety of grossular. Its distinctive "treacly" appearance comes from high refractive index and characteristic inclusions. Ceylon (Sri Lankan) material is considered finest.
What determines value in Grossular varieties?
Value varies by variety: Tsavorite values intense green color and clarity, Mali garnets prize unique pastels, while Hessonite values rich orange color and characteristic internal features.
Are treatments common in Grossular?
Unlike many gemstones, grossular varieties are rarely treated. Their colors are natural and stable, adding to their value. This particularly applies to Tsavorite, which offers natural green color without enhancement.
The specific gravity [?] for Grossular Garnet is 3.49, its refractive index [?] is 1.69-1.73, and its double refraction [?] is None.
History
Grossular gets its name from the Latin botanical term for "gooseberry" which it resembles in color and shape. Since the 1960s, it has been found in gem quality, mainly green. The gems, which are usually given a round or pear-shaped mixed cut, or occasionally a brilliant cut, are generally small, rarely exceeding one carat and never more than a few carats. If it is a good color (a strong, lively green), the gem can be in the top price bracket for secondary gems. This is especially true of the very rare examples weighing a few carats.
Industrial Usages
Clear, finely colored crystals of grossular are cut and sold as gemstones of excellent quality, though still relatively unpromoted in the trade. It is a mineral much in demand by museums and collectors.
Grossular is very similar in all respects to a recent artificial product of comparative structure, named green YAG (Yttrium Aluminum Garnet), which can be a good imitation, but does not contain silicon. Grossular itself is not produced synthetically.
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