A master of disguise. Spinel is the great impostor of gemstone world. In Burma where it is mined, spinel was recognised as a separate gem species to Ruby as early as 1587. In other countries the masquerade went on for hundreds of years. The Black Prince's ruby in the Imperial State Crown of England is in fact a very nice quality 170 carat red spinel.
Ironically, spinels are now rarer than the rubies they used to imitate. Odder still, they are also more affordable: in the gem world, being too rare can be a drawback because you need reliable supply to feed a reliable demand.
These days spinels are also made in a laboratory and many people might never come across a natural spinel.