Corundum
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Al2O3
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Second in hardness only to diamond, corundum is an aluminum oxide (Al2O3) with a Mohs hardness of 9. Its gem varieties are ruby, colored red by chromium, and sapphire, which covers all other colors. The mineral crystallizes in the trigonal system as tabular, prismatic, or bipyramidal crystals with an adamantine luster, a colorless streak, and no cleavage. Specific gravity ranges from 3.98 to 4.10. Corundum's extreme hardness makes it a widely used abrasive, and synthetic corundum serves in laser technology, while the finest natural crystals are cut as ruby and sapphire. Colors other than red span the full range of sapphire, from blue and yellow to pink and colorless.
Corundum is the second hardest natural mineral after diamond. Its gem varieties are ruby (red, colored by chromium) and sapphire (all other colors). Synthetic corundum is widely used as an abrasive and in laser technology.
| Crystal System | Trigonal |
| Mineral Class | 04 Oxides and Hydroxides |
| Mohs Hardness | 9.0 |
| Specific Gravity | 3.98-4.10 |
| Color | Red (ruby), blue (sapphire), various colors |
| Luster | Adamantine |
| Streak | Colorless |
| Cleavage | None |
| Fracture | Conchoidal to uneven |
| Transparency | Transparent |
| Crystal Habit | Tabular, prismatic, bipyramidal |
Mohs Hardness
| Mohs Hardness | Reference Mineral |
|---|---|
| 1 | Talc |
| 2 | Gypsum |
| 3 | Calcite |
| 4 | Fluorite |
| 5 | Apatite |
| 6 | Orthoclase |
| 7 | Quartz |
| 8 | Topaz |
| 9 | Corundum |
| 10 | Diamond |
Crystal System
| Property | Value |
|---|---|
| Crystal System | Trigonal (Rhombohedral) |
| Axial Relationships | a = b = c, α = β = γ ≠ 90° |