Ruby colored spinel has been confused with RUBY, the Black Prince's Ruby in the British Imperial State Crown being a famous example. The different colors of the mineral, caused by impurities, include various reds, yellow, blue, green, brown, and black, luster is vitreous. Gem spinels occur as crystals and pebbles in placer deposits, where they accumulate because they resist weathering.
Common spinel occurs in metamorphic rocks and as an accessory mineral of some basic igneous rocks.