Corundum vs Topaz
| Property | Corundum | Topaz |
|---|---|---|
| Formula | Al2O3 | Al2SiO4(F,OH)2 |
| Crystal System | Trigonal | Orthorhombic |
| Mineral Class | 04 Oxides and Hydroxides | 09 Silicates |
| Mohs Hardness | 9.0 | 8.0 |
| Color | Red (ruby), blue (sapphire), various colors | Colorless, yellow, blue, pink, orange |
| Luster | Adamantine | Vitreous |
| Streak | Colorless | Colorless |
| Transparency | Transparent | Transparent |
| Cleavage | None | Perfect {001} |
| Fracture | Conchoidal to uneven | Conchoidal to uneven |
Key Differences
- ◆ Corundum (hardness 9) is harder than topaz (hardness 8); corundum can scratch topaz.
- ◆ Topaz Al2SiO4(F,OH)2 is an orthorhombic silicate; corundum Al2O3 is a trigonal oxide.
- ◆ Topaz has perfect basal cleavage that can cause faceted gems to split; corundum has no cleavage.
- ◆ Corundum gem varieties (ruby, sapphire) are generally more valuable than most topaz.
- ◆ Topaz can be confused with sapphire when blue, but its cleavage and lower specific gravity distinguish it.