Alexandrite

Alexandrite is the colour-changing variety of chrysoberyl, the beryllium aluminate gem. A trace of chromium taking the place of some aluminium makes it soak up a narrow band of yellow light, so a fine stone looks green to bluish green by daylight and turns red to purplish red under incandescent lamps. Beryllium and chromium seldom turn up in the same rocks, and Wikipedia explains its rarity by the need for beryllium-rich pegmatite fluids to meet chromium-bearing rock; stones over three carats are very scarce. Wikipedia credits its discovery to Nils Gustaf Nordenskiöld, a mineralogist from Finland who first took it for emerald, and says it was named for the future Russian emperor Alexander II, though newer research points to Yakov Kokovin. Large stones were once thought to come only from the Ural Mountains, but it is also mined in Brazil, India, Madagascar, Tanzania and Sri Lanka, laboratories now grow synthetic stones, and much sold as synthetic alexandrite is really colour-change corundum or spinel. It is one of the June birthstones. Cunningham notes that it is rare and costly, and has it worn to draw luck and good fortune and used in love spells. Hall calls it a crystal of contrasts and associates it with intuition, willpower, self-respect, joy, creativity and manifestation.