Iolite is the trade name for gem cordierite. Iron gives it a violetish blue color.
Properties
| Refractive index | 1.542 to 1.551 |
|---|---|
| Birefringence | 0.005 to 0.018 |
| Optic character | Doubly refractive (biaxial) |
| Dispersion | 0.017 |
| Specific gravity | 2.61 |
| Hardness (Mohs) | 7.0 to 7.5 |
| Crystal system | Orthorhombic |
| Pleochroism | Strong: violetish blue, violet, pale yellow to colorless |
| Absorption spectrum | Not recorded |
Published values. They change a little from stone to stone. Our tools do not model iolite yet. Sources: GIA · International Gem Society · Gemdat
In more detail
Iolite is pleochroic: a violet stone shows light violet, dark violet and yellow-brown in different directions. Unlike sapphire, it cannot be heat treated to make its blue stronger, and it is not typically treated. It has perfect cleavage in one direction, so protect it from hard blows. Do not use steam or ultrasonic cleaners.
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.
- Sapphire: Sapphire is corundum of any color other than red. The word alone means a blue stone; other colors take a color word, such as yellow sapphire.
- 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.
- Tanzanite: 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.