Grossular

Grossular
"Mint" grossular garnets from Merelani Hills in Tanzania. 0.49 carats in total; stones 2.2 mm.
Image © supplied by Woodmansee* Gems

Grossular History & Etymology:

Grossular was first discovered in 1971 by Campbell R Bridges, a British gemologist and gem prospector. Grossular derives its name from the botanical name of the fruit gooseberry, Ribes Grossularium. This name has been derived due to the characteristic green coloring of the Grossular crystals which were found in the East Siberian region of Russia. The gemstones of Grossular belong to the garnet family.

Subsequently other varieties of Grossular were found and given various names according to their coloring. The most commonly found variety of Grossular is the Hessonite. Hessonite derives its name from the Greek word hesson which literally translates into inferior owing to the inferior hardness of Hessonite in comparison to the other varieties of Grossular.

Similarly, Tsavorite is another popular variety of Grossular crystals and was first discovered in the Tsavo National Park in Kenya. Hence the Tsavorite Grossular crystals derive their name in the honor of the National Park in which they were discovered. The name for the Tsavorite Grossular crystals was proposed by the then company president of Tiffany Sir Henry Platt.

Grossular Occurrence:

Grossular occur in many localities and local varietues. Hence Grossular are commonly found in the East Siberian region of Russia. The Grossular crystals found in this area are light green to dark green in color. The other varieties of Grossular crystals like the Hessonite and Cinnamon Garnet are typically found in USA, Brazil, Sri Lanka and Tanzania.

Tsavorite Grossular crystals are only found in the Tsavo National Park in Kenya where they were first discovered. This locality produces varies qualities of Tsavorite Grossular crystals which range from included crystals to gem quality crystals. Rosolite is another popular variety of Grossular crystals mostly found in Mexico while Viluite variety of Grossular crystals are found in the Wilui River Basin in East Siberia in Russia.

Grossular Properties:

Grossular is composed of calcium aluminum silicate and hence has a molecular weight of 450 grams. Grossular contains 26 percent calcium, 11 percent aluminum, 18 percent silicon and 42 percent oxygen. The dominant silicon in the composition of the Grossular mineral classifies it as silicate mineral.

Grossular displays isometric and hexoctahedral properties. Grossular is hence found in dodecahedra as well as trapezohedra formations in its natural form. The crystalline masses are characterized by massive striation and are at least 15 cm in size. The crystalline masses can be massive, granular and even compact in nature.

Grossular does not display any cleavages but shows irregular and sometimes uneven fracturing. This makes Grossular crystals quite brittle though they have a Moh’s hardness of 6.5. Grossular crystals are usually found with light to pale green coloring, pale yellow to golden yellow, greenish yellow, orange, red and pink coloring. Brownish red coloring is also observed in some Grossular crystals but is extremely rare.

The largest crystal of Grossular was found in Mexico and was 20 cm in size. It commanded a price which worked out at several hundred dollars per carat.

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