Ruby
A ruby gemstone
General
Category	Corundum variety, Oxide mineral
Formula (repeating unit)	aluminium oxide with chromium, Al2O3
Identification
Color	Red shades
Luster	Vitreous
References

A ruby is a kind of precious stone. It is often associated with emerald, sapphire and topaz. It is usually red, but can sometimes be other colors. It is formed of aluminum and oxygen (Al₂O₃), and that means it's a kind of corundum. A small amount of chromium makes the color.

Natural rubies