Corundum

Aluminum oxide and the second-hardest natural mineral. Red corundum is ruby; every other color is sapphire.

Corundum specimen — Photo: Ra'ike (see also Ra'ike on de.wikipedia, CC BY-SA 3.0, via Wikimedia Commons

How to identify corundum

Hardness 9 — scratches everything except diamond.
Barrel-shaped six-sided crystals.
Heavy for a non-metallic stone.
Often shows flat parting planes.

Often confused with

Garnet
Garnet is softer (7–7.5) and forms rounded dodecahedra.
Compare side by side
Kyanite
Kyanite is blue in flat blades and can be scratched along its length.
Compare side by side

Where it's found

Mogok, Myanmar
Sri Lanka
Madagascar
Yogo Gulch, Montana

Uses

Gems, emery abrasive, and synthetic sapphire for watch glass and camera lens covers.

Rock tumbling

Can be tumbled with care. Very slow — it takes weeks to round corundum.

Did you know?

Corundum is the Mohs reference for 9, and synthetic ruby has been made since 1902.

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