Opals

Opals are hydrated amorphous forms of silica, with a chemical composition approximating SiO2·nH2O. They contain water in their structure, typically between 3 and 21 percent, which influences stability and appearance. Opal lacks a true crystalline lattice, but some opals show a distinctive internal structure that diffracts light.

The most famous feature of opals is play-of-color, a spectrum of flashing colors visible as the stone is moved. This effect arises when submicroscopic silica spheres within the opal are arranged in a regular grid, causing light to diffract. Precious opal exhibits play-of-color, while common opal lacks this iridescence. Opals come in various forms and colors, including white, gray, and black. Fire opal is translucent to transparent with warm body colors, and boulder opal is opal found in ironstone host rock. Ethiopian opal includes crystal and semi-crystal varieties that can display vivid colors.

Formation occurs when silica-rich solutions fill voids in rocks such as sedimentary or volcanic units. As water content remains trapped, opal forms over long timescales. Australia dominates true opal production, accounting for the majority of the world supply, with notable sources in Mexico, Ethiopia, Honduras, and Nevada in the United States.

In jewelry, opals are valued for their color play and range of body tones but are relatively soft compared with many gems, with a Mohs hardness around 5.5 to 6.5. They are sensitive to dehydration and sudden temperature changes, so care involves gentle cleaning, storage with humidity, and avoiding prolonged exposure to heat or direct sun.

Synthetic and imitation opals exist, providing alternatives to natural stones. Opal is also associated with cultural symbolism and is the October birthstone in some traditions.