Renowned for their colorful iridescence, opals’ unique internal structure creates their colors by scattering light. Opal gemstones used in jewelry are relatively soft and fragile; they need protection from scratching, bumping, and abrupt temperature changes. Opal most commonly forms by the action of warm water on volcanic glass. It can also form at low temperatures, when water rich with dissolved silica mixes with sediment or enters rocks.
Opal deposits are found around the globe, but about 85 percent of the world’s gem opals are mined in Queensland and New South Wales, Australia. These opals come from a range of locations—Australia, Brazil, Honduras, Ethiopia, and the United States—and illustrate the gem’s diversity.