The name “opal” originates from the Greek word opallios, which meant “to see a change in colour.” The Roman scholar Pliny used the word opalus when he wrote about this gemstone’s kaleidoscopic “play” of rainbow colours that could simulate shades of any stone.
Opal’s characteristic “play-of-colour” was explained in the 1960s, when scientists discovered that it’s composed of microscopic silica spheres that diffract light to display various colours of the rainbow. These flashy gemstones are called “precious opals;” those without play-of-colour are “common opals.”
Opal’s classic country of origin is Australia. Seasonal rains soaked the parched Outback, carrying silica deposits underground into cracks between layers of rock. When the water evaporated, these deposits formed opal. Sometimes, silica seeped into spaces around wood, seashells and skeletons, resulting in opalised fossils.