Corundum

Chemical Classification: Oxide
Color: Typically gray to brown. Colorless when
pure, Chromium produces red (ruby) and
combination of iron and titanium produce blue (sapphire).
Streak: Colorless (harder than the streak plate)
Luster: Adamantine to vitreous
Diaphaneity: Transparent to translucent
Mohs Hardness: 9
Specific Gravity: 3.9 to 4.1 (very high for a nonmetallic mineral)