Polished rubies. Ruby is a red variety of the mineral corundum (aluminium oxide). Small amounts of chromium oxide, substituting for aluminium oxide, give it its red colour. Rubies are found in Burma, on the Malay Peninsula, and in Sri Lanka, Tanzania and Brazil. Natural rubies are highly prized as gemstones, but rubies can also be produced artificially, and such synthetic stones are used in lasers and watches.