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.
A ruby gemstone
|Category||Corundum variety, Oxide mineral|
|aluminium oxide with chromium, Al2O3|