What is Zoisite?
Zoisite is an orthorhombic calcium aluminium silicate whose green, pink and blue members make it one of the most attractive metamorphic minerals in the collector market.
Mineralogy and structure
Reference formula: Ca2Al3(SiO4)(Si2O7)O(OH). Zoisite has the ideal formula Ca2Al3(SiO4)(Si2O7)O(OH) and orthorhombic symmetry. Tanzanite is the blue-violet gemstone variety of zoisite and has its own Guide page; the link between the two records is intentional, not a duplicate colour entry. Zoisite is classified with silicates and has orthorhombic crystal symmetry. Typical hardness is 6–7 on the Mohs scale and specific gravity is approximately 3.1–3.4.
Colour, lustre and habit
Typical colours include green, pink, blue, grey, brown, colourless. The usual prismatic, granular, massive habit and vitreous, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Zoisite forms in or is associated with regional and contact metamorphism of calcium-rich rocks, with later alteration producing striking colour zoning. Important documented localities include Tanzania, Austria, Norway, Pakistan, India and the United States.
The mineral family behind tanzanite
Zoisite offers a strong collector story: natural crystal form, metamorphic origin and the famous blue-violet tanzanite connection. A well-documented specimen sells the geology as much as the colour.
Collecting and care
Avoid sudden thermal shock and strong acids. Blue tanzanite is commonly heat-treated, so disclose treatment and use a soft setting; preserve natural zoisite specimens from abrasion.
