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Grossular

Grossular is a calcium aluminum silicate garnet, Ca3Al2(SiO4)3, belonging to the garnet group and forming an isomorphous series with andradite and uvarovite known as the ugrandite series. The name derives from the botanical term "grossularia" for the gooseberry, first applied to pale green Siberian garnet, though grossular actually spans one of the widest color ranges in the mineral kingdom: colorless, cinnamon-brown, orange, pink, red, and vivid green. Well-known varieties include hessonite (cinnamon stone), tsavorite (chromium- and vanadium-bearing green gem garnet), rosolite or landerite (manganese-pink), and massive hydrogrossular, sometimes marketed as "Transvaal jade."

The single most celebrated locality is the Jeffrey Mine at Asbestos (Val-des-Sources), Quebec, Canada, whose rodingite zones produced world-famous gemmy orange, pink, and green-phantom grossular crystals alongside diopside, vesuvianite, prehnite, pectolite, and wollastonite. Classic hessonite gravels from Sri Lanka have supplied cut gemstones for centuries, while the Ala Valley in Piedmont, Italy is a historic European source of fine green and colorless crystals. Since its 1967 discovery by Campbell Bridges, tsavorite from the Merelani Hills in Tanzania and the Tsavo region of Kenya has become one of the most valuable green gem garnets, prized alongside emerald and tourmaline. Xalostoc and other skarns in Morelos, Mexico, are the type locality for pink rosolite, and notable American finds include skarn deposits in California's Sierra Nevada and the Vesper Peak area of Washington State.

Collectors seek grossular for its dodecahedral and trapezohedral crystal habit, glassy vitreous luster, and gemmy transparency rarely matched by other garnet species, along with a hardness of 6.5–7 that makes fine specimens durable as well as beautiful. Because it forms in calc-silicate skarns and rodingites at the contact of igneous intrusions with carbonate or ultramafic rocks, grossular is frequently found in striking association with diopside, wollastonite, vesuvianite, and scapolite, giving mineralogically rich, multi-species specimens. Whether as a gemstone rough, a cabinet crystal cluster from Quebec, or a bright green tsavorite crystal from East Africa, grossular remains a cornerstone species for garnet-group and calc-silicate skarn collections alike.

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