Tanzanite is the blue to purple variety of the mineral zoisite, and it contains vanadium. Its only commercial source is the Merelani area of northern Tanzania.
Properties
| Refractive index | 1.691 to 1.700 (GIA); full range 1.685 to 1.707 (LibreTexts, gemdat) |
|---|---|
| Birefringence | 0.008 to 0.013 |
| Optic character | Doubly refractive (biaxial) |
| Dispersion | 0.019 |
| Specific gravity | 3.35 |
| Hardness (Mohs) | 6 to 7 |
| Crystal system | Orthorhombic |
| Pleochroism | Strong: blue, violet to red-violet, yellow-green to brownish (removed or reduced by heating) |
| Absorption spectrum | Broad absorption in the yellow-green, 595 nm; weak band, 528 nm; weak band, 455 nm |
Published values. They change a little from stone to stone. Our tools do not model tanzanite yet. Sources: GIA · International Gem Society · LibreTexts Gemology · Gemdat
In more detail
Tanzanite is strongly pleochroic: a dichroscope shows blue, purple and sometimes yellowish brown. Most gem tanzanite is heat treated to remove the yellowish brown color. It has fair to poor toughness and cleavage, so protect it from hard blows. Never clean it with ultrasonic or steam cleaners.
On a grading report
LMHC wording: Species: (natural) zoisite. Variety: tanzanite. A note says tanzanite is commonly heated, and that this is not always detectable.
Related terms
- Pleochroism: Pleochroism is the property of a colored doubly refractive stone to show different colors in different directions. It happens because the stone absorbs its rays differently.
- Dichroscope: A dichroscope is a gem-testing instrument that shows a stone's two pleochroic colors side by side. In the calcite type, a calcite rhomb splits the light into two images of a small window.
- Heat treatment: Heat treatment is controlled heating of a gem to change its color or clarity.
- Cleavage: Cleavage is a mineral's tendency to split along flat planes in its crystal structure. A hard knock in a cleavage direction can split a stone.
- Toughness: Toughness is a gem's resistance to chipping and breaking. It differs from hardness, which is resistance to scratching.