Zoisite

Category  :  Silicate mineral
Hardness  :  6.5
Gravity  :  3.10-3.38
Reflection  :  1.70 / 1.69
Color  :  Gray, yellow, blue, green.
Chemical formula  :  Ca2Al3(SiO4)(Si2O7)O(OH)
Crystal system  :  Orthorhombic
Major varieties  :  Tanzanite (Blue Zoisite), Massive Green Zoisite

Its chemical formula is Ca2Al3(SiO4)(Si2O7)O(OH). Zoisite is named after the Slovene scientist Baron Sigmund Zois von Edelstein (Žiga Zois), who realized that this was an unknown mineral when it was brought to him by the mineral dealer Simon Prešern, who had discovered it in the Saualpe mountains (Svinška planina) of Carinthia in 1805. Zoisite was first known as saualpite, after its type locality. Transparent material is fashioned into gemstones while translucent-to-opaque material is usually carved into sculptural works. The latter is sometimes shot through with ruby crystals, which are completely opaque and unsuited to use as gems, yet are well colored and contrast strikingly against the green matrix of the zoisite.

Zoisite occurs as prismatic, orthorhombic (2/m 2/m 2/m) crystals or in massive form, being found in metamorphic and pegmatitic rock. Zoisite may be blue to violet, green, brown, pink, yellow, gray, or colorless. It has a vitreous luster and a conchoidal to uneven fracture. When euhedral, zoisite crystals are striated parallel to the principal axis (c-axis). Also parallel to the principal axis is one direction of perfect cleavage. Zoisite is somewhat higher than 6 in hardness and its specific gravity is between 3.10 - 3.38, depending on the variety. Zoisite streaks white and is said to be brittle. Clinozoisite is a more common monoclinic polymorph of zoisite.

Sources of zoisite include Tanzania (tanzanite), Kenya (anyolite), Norway (thulite), Switzerland, Austria, India, Pakistan, and Washington in the USA.

Tanzanite (Blue Zoisite)

This gem has a characteristic blue color, usually with a violet tinge. In lighter-colored specimens it is almost lavender. It can sometimes resemble cordierite, but this has much lower refractive indices and lower density. The stones have few inclusions; where present, these sometimes look like thin, parallel tubules. It is normally given a round or oval, mixed cut but the step cut is also used.

Massive Green Zoisite

An ornamental material consisting of crystalline aggregates or green zoisite with ruby inclusions, also discovered recently in Tanzania. The ground color is bright green, forming a striking contrast with the isolated crystals of ruby which are bright red, some tens of millimeters in size, and evenly distributed throughout the mass.

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