Spinel is very hard, 7.5 to 8 on the Mohs scale, and commonly forms chunky octahedral crystals. You'll typically find it in metamorphosed limestones and low-silica plutonic rocks, often accompanied by corundum. Its color ranges from clear to black and almost everything in between, thanks to the wide range of metals that can partially replace the magnesium and aluminum in its formula. Clear red spinel is a significant gemstone that can be confused with ruby—the famous jewel known as the Black Prince's Ruby is one.
Geochemists studying the mantle refer to spinel as a crystallographic structure, like that of the mineral spinel. For instance, olivine is said to adopt the spinel form at depths greater than about 410 kilometers.
