Grossular garnet: The mineral Grossular information and pictures

About

Grossular is member of the Garnet group, and is its most varicolored form, occurring in almost all colors except for blue. Grossular is often dark in color, but it also forms the lightest colored forms of Garnet, and can even be white or colorless. Although not as famous as Almandine and Pyrope, Grossular has some important gem varieties. These include the rare emerald-green Tsavorite and the orange to orange-brown Hessonite.

Grossular forms a solid solution series with Andradite, and can be virtually indistinguishable from it in localities where they both occur together. In some localities such as Mali, a distinction between these two Garnets is sometimes too complex, and a specimen may just be called "Andradite/Grossular since its full identification is lacking. Grossular is named for its color resemblance to gooseberries, which are scientifically known as ribes grossularium.

Crystal Forms & Aggregates

Usually in sharp dodecahedral crystals and dense dodecahedral aggregates. Crystals often have growth patterns, etchings, and striations. Occasionally in trapezohedral crystal or trapezohedral-dodecahedral modifications. Also massive and in dense growths of tiny crystals.

Other ID Marks

Occasionally fluorescent in ultraviolet light.

Striking Features

Crystal forms and common association with Vesuvianite

Environment

In contact metamorphic rocks in skarns and hornfels and in asbestos Serpentine deposits.

Varieties

✓ common · ✗ uncommon

Hessonite— Orange to orange-brown, transparent variety of Grossular Garnet.
Mali Garnet— Garnet from the African country of Mali that ranges from green to yellow to brown (though most often a greenish-yellow). The deposit of these Garnets was discovered in Mali in 1994, and their scientific classification is not clearly identified; they can be either Grossular or Andradite depending on their chemical composition. X-ray analysis has determined most of these Garnets to be an intermediary form of the Grossular / Andradite series, though closer in chemical structure to Grossular. Although this is a relatively new term, it has become extensively used in the gem trade.
Tsavorite— Rare emerald-green, transparent variety of Grossular Garnet from Kenya and Tanzania in Africa. Tsavorite is a relatively recent gemstone, with the term "Tsavorite" first being coined by Tiffany and Co. in the 1970's. Though its name is not historically significant and has been only recently coined, this term has become widely used and accepted in the gemstone industry.
For more detailed see the gemstone section on Tsavorite.
African Jade— Synonym of Transvaal Jade.
Caswellite— Grossular pseudomorph after mica from Franklin, New Jersey, that retains a foliated appearance.
Hydrogrossular— Synonym of Hydrogarnet
Leuco-garnet— Colorless, transparent variety of Grossular Garnet.
Raspberry Garnet— Raspberry-red Grossular Garnet from the Lake Jaco area in Sierra de la Cruz, Coahuila, Mexico.
Rosolite— Pink to raspberry-red variety of Grossular Garnet from the Lake Jaco area in Sierra de la Cruz, Coahuila, Mexico.
Transvaal Jade— Massive veins of opaque Grossular Garnet that resembles Jade. It is white to light green in color, and may be colored in a white, green, and pink combination. See Hydrogarnet for more details.

Uses

Although slightly softer than other Garnets, Grossular Garnets have good color and transparency and make good gemstones, though large cuttable crystals are uncommon. The most prized Grossular gemstone is the rare and valuable emerald-green Tsavorite. Hessonite is occasionally used as a gemstone, as well as the light green to yellowish-green variety. The massive, vein-like material usually categorized as Hydrogrossular is occasionally carved into ornaments and cabochons.
See the gemstone section on Grossular, Tsavorite, and Garnet for more information.

Grossular is a valuable mineral to collectors, and good transparent specimens can demand a high price.

Noteworthy Localities

Grossular has many good localities worldwide. This list describes some of the better-known locations. Italy is famous for its many Grossular occurrences, especially the Hessonite variety; noteworthy Italian localities include the Val D'Ala; where it occurs with outstanding Diopside; Condove, Val D'Susa; Bellecombe, in the Val D'Aosta; Carboneri, in the Val D'Pellice; and Urbe, Savona Province. Other European localities are Felskinn, Saas Fee, Wallis, Switzerland; and the Ocna de Fier-Dognecea District, Banat Mts, Romania.

The desirable deep green Tsavorite variety comes from the Scorpion Mine, Tsavo National Park, Kenya (hence its name); and the Merelani Hills, Arusha, Tanzania. Large brown floater crystals come from the Kayes Region, Mali. Light green crystals, sometimes in a rare trapezohedral form, come from the the Vilyui River Basin, Yakutia, Russia.

In Mexico, exceptionally large crystals and clusters are found near Lake Jaco, in Sierra de la Cruz, Coahuila. Most crystals are beige to light-greenish-gray, though a very attractive raspberry-red form is found there as well.

Perhaps the most outstanding locality of Grossular is the Jefferey Mine in Asbestos, Québec, Canada. This mine was famous for its lustrous transparent brown Hessonite crystals, as well as a chromium-rich deep green form. Other occurrences in Québec include the Thetford Mines and nearby Black Lake, which produce deep green chromium-rich crystals; and the Orford Nickel mine, St-Denis-de-Brompton, where electric green microcrystals embedded on Diopside were found.

In the U.S., some of the finest Grossular crystals have come from the Belvidere Mountain quarries, Lowell/Eden, Orleans & Lamoille Cos., Vermont, in sharp transparent crystals. Orange-brown Grossular comes from Maine at the Pitts-Tenney Quarry, Minot, Androscoggin Co.; and at Sanford, York Co. Small Grossular crystals embedded in large massive Grossular matrix can be found at West Redding, Fairfield Co., Connecticut. Other U.S. localities include the Hunting Hill quarry, Rockville, Montgomery Co., Maryland; Vesper Peak, Sultan Basin, Snohomish Co., Washington; Bishop, Inyo Co., California; and Havila, Kern Co., California. Light greenish-gray floater crystals come from the Wah Wah Mountains in Beaver Co., Utah.

Common Mineral Associations

Vesuvianite, Calcite, Diopside, Serpentine, Clinochlore, Clinozoisite, Wollastonite, Tremolite, Quartz

Distinguishing Similar Minerals

Spessartine - Very difficult to distinguish without complex methods, though Spessartine usually forms in trapezohedral crystals whereas Grossular forms in dodecahedral crystals.
Andradite - Very difficult to distinguish without complex methods.
Uvarovite - Usually darker green and in denser aggregates, and found only in chromium-rich deposits.
Tourmaline - Lighter in weight, forms different crystals.
Vesuvianite - Forms different crystals.