Ruby, variety of corundum, aluminium oxide, formula Al₂O₃.

Crystal system: Trigonal.
Hardness: 9
Density: 3.9 to 4.1
Colour: Red.
Streak: White.

Ruby is the name given to the red variety of corundum, aluminium oxide. Colourless corundum is called leucosapphire. Other colours of corundum are blue (sapphire), yellow, violet and blue-green. The red colour of ruby is caused by chromium (Cr) impurities. Ruby is used as a precious stone and is usually facetted. In the days of mechanical watches it was also used as a bearing material because of its hardness and hence resistance to wear. Occurrence is rare.

The picture shows a terminated crystal of pinkish red ruby from Nuristan, Afghanistan.