For centuries, black spinel was mistaken for other dark stones. Gems set into royal crowns as ""black diamond"" were later identified as spinel.

Mohs hardness of 8. Short of diamond at 10, but second only to sapphire at 9. Formed from magnesium and aluminium oxides, its crystal structure accounts for that toughness.

Mined in Sri Lanka, Myanmar, and Tanzania, frequently found alongside other gemstones—which is exactly why it was always taken for something else. Recognition under its own name came only recently.