Corundum

Chemical composition -- Aluminum oxide.

Color -- almost all colors. Medium to dark red stones are known as ruby; other colors, sapphire.

Optics -- R.I. 1.76-1.77.

Durability -- Hardness 9. After diamond, the next hardest gem material. Tough, but still subject to chipping and abrasion. Stones worn regularly in rings will periodically need repolishing to remove minor chips and pits.

Crystal structure -- Hexagonal .

Specific Gravity -- 4.0.

Sources -- Many locations, but the best known are in southeast Asia, Australia, and southern Africa. In the U.S., Montana and North Carolina are well known sources.