Zoisite is a mineral with a hardness of 7 out of 10 on the Mohs scale of mineral hardness [?]. These Orthorhombicly structured gems are made of calcium aluminum hyrdroxysilicate, their full chemical compound being Ca2(Al,OH)Al2(SiO4)3.
Zoisite is a hydrous calcium aluminum silicate in the Epidote group, with an orthorhombic crystal system, three crystal axes at right angles to each other, all of varying lengths.
It appears in elongated, prismatic crystals, with fine parallel lines on the prism faces and usually poorly terminated. It also appears frequently in formless grains, poorly-defined crystals, rodlike aggregates and granular masses.
It comes in the colors white, blue, pale green, sometimes brilliant colors like pink (manganese-bearing thulite) and violet blue (tanzanite containing chromium and strontium).
It is hard, heavy, with distinct pinacoidal cleavage, two faces exactly parallel. It is usually transparent, with vitreous luster, is insoluble in acids, and fuses fairly easily into a blistered white glass.
Zoisite occurs in high-temperature and high-pressure metamorphic rocks (eclogites, granulites, and some varieties of gneiss). It is also in hydrothermal veins, associated with sulfides, and as an alteration product of calcic plagioclases in extrusive rocks. In very rare cases, it is found in metamorphic contact aureoles in impure calcareous (calcium-carbonate) rocks. It is sometimes present as an accessory mineral in rocks.
The varieties of zoisite are:
Tanzanite (blue zoisite)
This is the blue variety of zoisite discovered in Tanzania in 1967, but this deposit is now exhausted. Tanzanite is produced from material occurring in the Merelani Hills, Lalatema Mountains, Tanzania. Produced, because most blue tanzanite is now made by heat treatment of brownish zoisite. It has a characteristic blue color, usually tinged with violet. In good quality tanzanite, the color is ultramarine to sapphire blue; in artificial light, it is more like amethyst violet. In lighter-colored specimens, it is almost lavender. This is the principal color, visible from the table facet in cut stones, because another characteristic of tanzanite is strong pleochroism from violet-blue to violet, grayish or greenish, all of which can be seen depending on what angle you view it. When present, inclusions sometimes look like thin, parallel tubes. It is normally given a round, or oval, mixed cut, but the step cut is also used. Its moderate hardness doesn't make it a good stone for rings, which are prone to knocks and abasion. Tanzanite cat's eyes have also been found.
The dense rose-red to pink variety of zoisite. First found in Norway, near a place called Thule, it has also been found in Western Australia and South Africa. It is used as a cabochon and as an ornamental stone. It is also known as rosalite, but can be confused with rhodonite.
Anyolite or Massive green zoisite
This is an ornamental material consisting of crystalline aggregates of green zoisite with ruby inclusions, also discovered in Tanzania. Its appearance is so unique as there is nothing else like it in the mineral world. The ground color is bright green, a striking contrast with the isolated crystals of ruby which are bright red (but nontransparent), which are strewn evenly throughout the mineral mass. The overall effect is very pleasing, and this material is sawn, turned, sculpted, and polished, mainly into items such as ashtrays, boxes, and small carvings. The price of items made of massive green zoisite depends on the quality of the workmanship. In Tanzania's native language, it is called Masai anyolite.
Zoisite comes from Sausalpe (Austria), Wyoming (USA), Kenya and Tanzania, and from the Yourma Mountains (Urals, Russia) in fine crystal aggregates. All tanzanite comes from the Tanga province (Tanzania), though large, clear crystals are becoming increasingly rare. Thulite is found in many manganese deposits, including those of Telemark (Norway), and Tennessee and South Carolina (USA). It is also found at Pra Isio (Sondrio), and Vipiteno, Bolzano (Italy).
Zoisite, named after the collector Zois, was first found in the Sau-Alp mountains of Austria in 1805. It was originally called saualpite, and has only recently been considered a gemstone.
Zoisite is of interest mainly to scientists and collectors.
Tanzanite, made famous by Tiffany & Co. who gave the variety its name, is an important gemstone in its own right, and is not considered simply a replacement for blue sapphire. Tanzanite jewelry should never be cleaned with ultrasonic cleaners commonly used by jewelers and watchmakers, because of its very low resistance to ultrasound. Stones are known to suffer irreparable damage afterwards.
Because of its attractive color, its rarity, and the publicity which greeted its discovery, the value of tanzanite is quite high, little less than that of the violet-blue sapphires it resembles. But it is rarely seen on the market and is very much a collector's item.