Zoisite

Calcium Aluminum Silicate Hydroxide
Ca₂Al₃(SiO₄)₃(OH) Zoisite, like all epidote minerals, is a structurally complex mineral having both single silicate tetrahedrons, SiO₄, and double silicate tetrahedrons, Si₂O₇. The formula of zoisite could be expressed in a such a way so as to reflect this organization; Ca₂AlOAl₂(SiO₄)(Si₂O₇)(OH).

Zoisite has been known for nearly two centuries as as a sometimes ornamental stone of limited distribution. Only in 1967 was the blue gemstone variety found in Tanzania. The variety was named Tanzanite and was a surprise to minerologists and gemologists alike in that it had come from a very ungemstone-like mineral. The blue-lavender color of tanzanite is unique and sets it apart from the other gemstones.

Besides tanzanite, zoisite has produced other attractive specimens that are of interest to collectors. A pink variety called thulite is usually massive and used for beads and cabochons. A brilliant green variety is associated with medium grade rubies and is quite popular as an ornamental stone. The red rubies are often distorted and irregularly spread throughout the sea of massive green zoisite. It is one of the most colorful of ornamental stones and competes well with the popular pink tourmaline and lavender lepidolite of California.

Hardness	Associated Minerals	Streak	Colour	characteristics	Luster	Field Indicators
6 - 7	calcite biotite hornblende quartz corundum andradite garnets and other metamorphic minerals	white	variable but well known as green, blue to violet and pink to reddish in color, Also grey, white or brown	striated crystals common, lengthwise. Also strongly pleochroic with an index of refraction of 1.68 - 1.72	vitreous	only one direction of cleavage, associations, color, pleochroism and hardness