What is Tanzanite?
Tanzanite is the blue to violet gem variety of zoisite, prized for strong pleochroism, saturated colour and its exceptionally restricted source near Merelani in Tanzania.
Mineralogy and structure
Reference formula: Ca2Al3(SiO4)(Si2O7)O(OH). Tanzanite is a variety of zoisite rather than a separate mineral species. Its orthorhombic structure is represented here by a labelled representative zoisite model from COD 9000179; heating commonly changes brownish material to blue-violet, so treatment should be disclosed. Tanzanite is classified with silicates and has orthorhombic. Typical hardness is 6–7 on the Mohs scale and specific gravity is approximately 3.3–3.4.
Colour, lustre and habit
Typical colours include blue, violet-blue, violet, burgundy, brown, green. The usual prismatic, columnar, granular habit and vitreous lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Tanzanite forms in or is associated with metamorphic rocks and hydrothermal-bearing graphite-rich settings in a restricted gem district. Important documented localities include Merelani Hills near Mount Kilimanjaro, Tanzania.
A rare blue-violet gem with strong pleochroism
Tanzanite is commercially distinctive because its blue-violet colour, pleochroism and single mining region create a clear provenance story. Colour, face-up orientation, clarity, size and treatment history strongly affect value.
Collecting and care
Protect from sharp impact and sudden thermal shock. Warm soapy water is generally safe; avoid ultrasonic and steam cleaning, especially for fractured or treated stones.
