Corundum vs Spinel
| Property | Corundum | Spinel |
|---|---|---|
| Formula | Al2O3 | MgAl2O4 |
| Crystal System | Trigonal | Cubic (Isometric) |
| Mineral Class | 04 Oxides and Hydroxides | 04 Oxides and Hydroxides |
| Mohs Hardness | 9.0 | 7.5-8.0 |
| Color | Red (ruby), blue (sapphire), various colors | Red, blue, green, black, colorless |
| Luster | Adamantine | Vitreous |
| Streak | Colorless | White |
| Transparency | Transparent | Transparent |
| Cleavage | None | None |
| Fracture | Conchoidal to uneven | Conchoidal |
Key Differences
- ◆ Corundum (hardness 9) is harder than spinel (hardness 7.5–8); corundum is the second hardest natural mineral.
- ◆ Both produce red gemstones — ruby (corundum) and red spinel — that were historically confused for centuries.
- ◆ Corundum is a trigonal oxide (Al2O3); spinel is a cubic oxide (MgAl2O4).
- ◆ Corundum has no cleavage; spinel also has no cleavage — both show conchoidal fracture.
- ◆ Spinel occurs in metamorphosed limestones; corundum forms in a wider range of igneous and metamorphic rocks.