Zoisite: Gemstones Information
Zoisite is Ca2Al3(SiO4)(Si2O7)O(OH) and forms transparent to translucent crystals or compact masses of the orthorhombic system. Crystals areprismatic and deeply striated; colours are white-grey, greenish brown, greenish grey, pink, blue and purple. The fine transparent blue tanzanite variety is produced by heat treatment of brownish material and so far has not been found in this colour. The hardness is 6–7 and the SG for tanzanite 3.35. The RI for the alpha, beta and gamma rays in tanzanite is 1.692, 1.693 and 1.700, biaxial positive with birefringence 0.009. Tanzanite has very pronounced pleochroism, some crystals showing ruby red, emerald green, or gold, and blue colours. Tanzanite shows an absorption band centred at 595 nm with fainter bands at 528 and 455 nm; there may be faint lines in the red.Zoisite, a member of the epidote group, occurs in medium-grade regionally metamorphosed crystalline schists formed from igneous,sedimentary or metamorphic rocks relatively high in calcium. Tanzanite is produced from material occurring in the Merelani Hills, Lalatema Mountains, Tanzania. Some examples of the unheated crystals have been faceted to give transparent brown specimens. Transparent green crystals from Turmik, Skardu district, northern areas of Pakistan, have also been faceted. Massive dark pink to yellow zoisite is sometimes fashioned and is sometimes known as thulite. Ruby crystals in green zoisite have beennamed anyolite; the source is the Longido area of Tanzania.