Crystal & mineral guide
Tanzanite
Ca₂Al₃(SiO₄)₃(OH) (zoisite)
Blue-violet zoisite known from a single mining district in Tanzania.
Identify Tanzanite with RockScanAbout Tanzanite
Tanzanite shows strong trichroism (blue/violet/burgundy). Most commercial stones are heat-treated to remove brown and enhance blue-violet. Perfect cleavage and modest hardness require protective settings. Single-source supply supports strong branding in retail.
Tanzanite belongs in a crystal identifier workflow because photo cues — color zoning, crystal habit, and surface luster — combine with measurable traits like Mohs 6–7 and specific gravity 3.25–3.36. Use RockScan’s camera to get a first-pass ID, then confirm with the checklist below.
Tanzanite properties
| Mohs hardness | 6–7 |
|---|---|
| Crystal system | Orthorhombic |
| Luster | Vitreous |
| Color | Blue to violet (trichroic) |
| Composition | Calcium aluminum hydroxy silicate (zoisite variety) |
| Formation | Metamorphic graphite-bearing gneisses |
| Occurrence | Merelani Hills, Tanzania (essentially single-source) |
| Specific gravity | 3.25–3.36 |
| Cleavage | Perfect |
How to identify Tanzanite
- Strong trichroism under dichroscope
- Perfect cleavage — handle carefully
- Heat treatment nearly universal in trade
Collector notes
- Named and marketed by Tiffany & Co. in 1968
- Found in commercial quantities only in northern Tanzania
Value context: Saturated blue-violet clean stones over 5 ct bring premiums.
Tanzanite FAQ
How do you identify Tanzanite?
Tanzanite (Ca₂Al₃(SiO₄)₃(OH) (zoisite)) is typically recognized by blue to violet (trichroic) color, vitreous luster, Mohs hardness 6–7, and orthorhombic crystal system. RockScan can suggest Tanzanite from a photo and surface these checks on the result card.
What is Tanzanite made of?
Tanzanite composition: Calcium aluminum hydroxy silicate (zoisite variety). It forms via metamorphic graphite-bearing gneisses and is found in Merelani Hills, Tanzania (essentially single-source).
Is Tanzanite valuable?
Saturated blue-violet clean stones over 5 ct bring premiums.
