Orthorhombic
Tanzanite is the trade name for the violet-blue gem variety of zoisite, found at exactly one locality on Earth: the Merelani Hills near Arusha, Tanzania. Discovered in 1967 and marketed worldwide by Tiffany & Co., tanzanite is one of the great gem stories of the late 20th century.
Occurrence
Unique locality: Merelani Hills, Manyara Region, Tanzania. Tanzanite forms in graphite-rich pegmatite-hosted gem pockets within metamorphic gneisses of the Mozambique Belt. Most rough is heat-treated (~600°C) to enhance the violet-blue color from natural brown-yellow tones.
Identification
Strong trichroism (blue, violet, brownish-yellow along crystal axes) + Tanzanian pedigree + perfect cleavage. Distinguish from sapphire (harder, no cleavage, isotropic blue) and iolite (lower density, weaker color).
Collector Notes
The Tanzanian government has restricted rough exports since the early 2000s; supply tightening has driven tanzanite into top-tier gem rarity territory. Crystallized rough specimens (uncut crystals on matrix) are increasingly valuable.
Quality criteria — what to look for
Gemological optical data
- Refractive index
- 1.691–1.700
- Birefringence
- 0.009
- Dispersion
- 0.030