Corundum (Ruby & Sapphire)
Al₂O₃ · Trigonal · also called ruby, sapphire, emery
Corundum is aluminium oxide and the second-hardest natural mineral. Most natural corundum is dull grey-brown and industrial rather than gem quality, but the hardness test is decisive: if your crystal cuts a clean groove in quartz, you have a very short list of candidates.
Properties
- Mohs hardness
- 9
- Streak
- white, colorless
- Lustre
- adamantine, vitreous
- Colour
- Red corundum is ruby; every other colour is sapphire, including the colourless and grey material that makes up most of what is actually found.
- Cleavage
- No cleavage, though basal parting is common and can look like cleavage.
- Fracture
- Conchoidal to uneven
- Specific gravity
- 3.95–4.1 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Some specimens fluoresce
Tests that confirm corundum
- Hardness 9 — it scratches quartz, topaz, and every common mineral except diamond. Nothing else you find casually will do this.
- Barrel-shaped or tapering hexagonal crystals with flat ends.
- Distinctly heavy for a transparent mineral.
What corundum gets confused with
Each of these is settled by one test. Run that test first.
Hardness. Corundum scratches garnet decisively; garnet cannot mark corundum.
Spinel is softer (8) and forms octahedra rather than barrels. Historically the two were confused for centuries.
Where it is found
Metamorphosed aluminium-rich rocks, some alkaline igneous rocks, and gem gravels in Sri Lanka, Myanmar, Madagascar, and Montana.
Is it worth anything?
Gem-quality ruby and sapphire are among the most valuable materials on Earth by weight. Ordinary opaque corundum is worth very little. Assume opaque unless proven otherwise.