One of the best-known physical properties of topaz is its hardness. It has a hardness of 8 on the Mohs hardness scale, making it the hardest silicate mineral. Every student who takes a physical geology course learns about the exceptional hardness of topaz. Diamond, corundum, and chrysoberyl are the only commonly known minerals that are harder than topaz.
Topaz occurs in a wide range of colors. The most valuable colors for use in jewelry are natural pink, orange, red, purple and blue. These colors are very rare.
The most common natural colors of topaz are colorless (often called “white topaz”), pale yellow, and brown. While these colors are not often used in jewelry in their natural state, they can be treated in a variety of ways to produce colors that are much more desirable.
When allowed to grow in an unrestricted cavity, topaz forms orthorhombic crystals, often with striations that parallel the long axis of the crystal. It also has a distinct basal cleavage that breaks to form vitreous fracture surfaces perpendicular to the long axis of the crystal. This cleavage makes topaz a more fragile gemstone than its hardness of 8 would imply. Topaz is very hard, but it has a low tenacity, which means it chips and cleaves easily.
Topaz has a specific gravity that ranges between 3.4 and 3.6. This is surprisingly high for a mineral composed of aluminum, silicon, and gaseous elements. This high specific gravity causes it to be concentrated into placer deposits by stream currents.
| Physical Properties of Topaz | |
| Chemical Classification | Silicate. |
| Color | Natural colors include: colorless, yellow, orange, brown, red, pink, blue, green. Occurs in a wide range of treated colors, most often blue. |
| Streak | Colorless – harder than the streak plate. |
| Luster | Vitreous. |
| Diaphaneity | Translucent to transparent. |
| Cleavage | Perfect basal cleavage. |
| Mohs Hardness | 8 |
| Specific Gravity | 3.4 to 3.6 |
| Diagnostic Properties | Hardness, prismatic crystals, sometimes striated, cleavage, specific gravity. |
| Chemical Composition | Al2SiO4(F,OH)2 |
| Crystal System | Orthorhombic. |
| Uses | Gemstone, Mohs hardness index mineral. |