In 2016, jeweler Olivia Young of Ouroboros sold a red spinel ring for £6,000 ($7,700). Last year, the client’s insurance company paid out £30,000 ($38,400) to replace it.

Spinel comes in a variety of colors, such as red, pink, orange, purple, blue and green. Due to this range, spinel has been confused with other gems. Some well-known rubies have turned out to be ...