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Tourmaline

A polished tourmaline from India gleams. Cut in a convex form known as a cabochon, this red tourmaline could also be called a rubellite because of its red coloration. An Egyptian legend explains that tourmalines come in so many different colors because they passed through a rainbow on the journey from the Earth's center, but scientists know that a tourmaline's chemical composition determines its color.

Photograph by Hemis/Alamy