This stone, whose magnificent shimmering highlights (or “play of color”) are caused by the diffraction of light by an orderly arrangement of minute spheres of silica, offers many varieties. Those used in jewelry are “precious”, as opposed to “common”. Harlequin opals are a milky white color and reflect bright flashes of light in every conceivable color. Fire opals, as their name suggests, are red with red-orange highlights. The rarest variety, the black opal, is very dark with multi-colored highlights. Opal has been known to Man since the dawn of history, if the etymology of its name (the Sanskrit word upala meaning “precious stone”) is any indication. Ancient and Arabic civilizations believed it had protective virtues, and thus used it to create endless amulets, talismans and jewelry. In contrast, nineteenth century France accused the opal of bringing bad luck. Opal was brought back into favor by jewelers in the Art Nouveau period. In 1903 Louis Cartier created a magnificent heart-shaped opal pendant for the celebrated Australian soprano Nellie Melba.Hydrous silicon dioxide. Hardness: 5.5 to 6.5. Birthstone of October. Australia, Mexico, Brazil, United States.