Formula: Al₂O₃

Species: Oxides

Colour: Red

Hardness: 9

Crystal System: Trigonal

A red, gem variety of corundum. The red colour is caused by minor amounts of trivalent Cr replacing Al in the crystal structure. In traditional gemmological terms, ruby has to be blood-red and of clear, facetable quality to justify the name, however in wider usage any corundum with a red or reddish colour has attracted the name ‘ruby’, and this name is usually applied in this way by mineral collectors. Pure Cr2O3 occurs in nature as the mineral eskolaite which is green.