Topaz is a gem mineral with an exceptionally wide color range. Impurity elements or defects in its crystal structure cause its color.
Properties
| Refractive index | 1.619 to 1.627 (GIA); full range 1.606 to 1.644 by F/OH content (LibreTexts, gemdat) |
|---|---|
| Birefringence | 0.008 to 0.010 |
| Optic character | Doubly refractive (biaxial) |
| Dispersion | 0.014 |
| Specific gravity | 3.53 (GIA); 3.50 to 3.57 by color (IGS) |
| Hardness (Mohs) | 8 |
| Crystal system | Orthorhombic |
| Pleochroism | Distinct: bright blue, pale rose, colorless |
| Absorption spectrum | Weak line in heat-treated pink stones; otherwise the spectrum is not diagnostic, 682 nm (Cr) |
Published values. They change a little from stone to stone. Our tools do not model topaz yet. Sources: GIA · International Gem Society · LibreTexts Gemology · Gemdat
In more detail
Chromium causes the natural pink, red and violet colors. Blue topaz is hardly ever natural: treatment of colorless topaz gives the blue. A thin metallic oxide coating is another treatment, and abrasive cleaners can remove it. Topaz has cleavage and poor toughness, so protect it from hard blows, heat, steam and ultrasonic cleaners.
Related terms
- Aquamarine: Aquamarine is the green-blue to blue variety of the mineral beryl. Emerald and morganite belong to the same species.
- 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.
- Irradiation: Irradiation is exposing a gem to artificial radiation to change its color. Heating sometimes follows, to modify the color further.
- Coating: Coating is a thin film, paint or ink on the surface of a gem that changes its color or look.
- Heat treatment: Heat treatment is controlled heating of a gem to change its color or clarity.