Opal is an amorphous form of silica related to quartz. Revered as a symbol of hope fidelity and purity, opal was dubbed the Queen of Gems by the ancient Romans because it encompassed the colors of all other gems. Opal, the birthstone for October, is prized for its unique play of color, the ability to diffract light into flashes of rainbow color. The most common is white opal and the most valued is black opal. Mined primarily in Australia, Mexico, and the United States opals are cut into rounded or free form cabochons that enhance their color play.