Alexandrite

A rare variety of chrysoberyl that changes color with the light: green to bluish-green in daylight, red to purplish-red under indoor lighting; "emerald by day, ruby by night." The original Russian deposits are essentially exhausted, so most stones now come from Brazil, Sri Lanka and East Africa, and fine natural material is expensive. At 8.5 on the Mohs scale it wears well every day. Lab-grown alexandrite is common, and color-change sapphire is often mistaken for it.