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The opal gemstone is a hydrated amorphous form of silica, meaning it contains water within its structure. Its formation typically occurs at relatively low temperatures within sedimentary rocks, often in cavities or fissures. The iconic play-of-color, a characteristic shimmering rainbow of hues, is caused by the diffraction of light through the microscopic spherical silica spheres that make up the opal. The size and arrangement of these spheres determine the colors displayed and the intensity of the play-of-color.

Opal is found in numerous locations around the world, with Australia being the most significant producer, responsible for over 90% of the world's supply. Other notable sources include Ethiopia, Mexico, Brazil, and the United States. The gemstone is classified into various types based on its appearance and origin. Common opal lacks the play-of-color and is often opaque. Precious opal, on the other hand, exhibits the desirable play-of-color. Within precious opal, distinctions are made between different varieties such as white opal, black opal (highly prized for its dark body color), boulder opal, and fire opal.

The hardness of opal on the Mohs scale ranges from 5.5 to 6.5, making it a relatively soft gemstone that requires careful handling to avoid scratching or chipping. Due to its hydrated nature, opals can lose moisture and crack if exposed to extreme temperatures or prolonged dryness. Therefore, proper storage and care are essential to maintain their beauty and integrity. Historically, opal has been associated with various myths and legends, often seen as a symbol of hope, purity, and good fortune.