Corundum is a common mineral found in metamorphic and igneous rocks. It is also a popular and valuable gem.
Ruby is the gem variety of red/pink corundum that is colored by trace amounts of chromium (Cr).
Sapphire is the gem variety of corundum colored by iron (Fe) or titanium (Ti). Though sapphire comes in every color of the rainbow, it is commonly blue.
Both ruby and sapphire are known to display asterism. This is a property where included needles of rutile align themselves with the axes of the hexagonal corundum yielding a six-armed star when viewed down the long c-axis.
Emery is black, granular corundum that is usually mixed with magnetite or hematite and used as an abrasive.
| Al2O3 |
|
| Oxide |
|
| Hexagonal |
|
| -3 2/m |
|
| 9 |
|
| 4.02 |
|
| {0001}, {10-11} |
|
| Conchoidal |
|
| Blue, pink, brown, colorless, orange, green, purple, yellow |
|
| White |
|
| Adamantine, vitreous |
|
| Transparent/translucent |
|
| Granular |
|
| Notable Localities: |
Thailand; Sri Lanka; Cambodia; Kashmir, India; Queensland, Australia; Montana |
| Uses: |
Cut as a gem, abrasives |
| Chlorite, muscovite, olivine, magnetite, spinel, kyanite, diaspore |
Specimens:
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