Is Opal a Mineral or a Rock?

Opal is an amorphous form of silica, technically classified as a mineraloid. Unlike most minerals, opal does not have a crystalline structure. Instead, opal is made up of silica spheres in an irregular arrangement suspended in a matrix of silica and water. This unique structure is what gives opal its optical properties and makes it so highly prized as a gemstone.

Opal was long considered a mineral in its own right, but in the 1960s, the International Mineralogical Association reclassified opal as a mineraloid due to its lack of an organized crystalline structure. So opal is neither a true mineral nor a rock, but rather falls into the category of mineraloid.

How Opal Forms

The silica contained in opal originates from the leaching of silica from volcanic rocks or siliceous sediments. As this silica-rich water seeps through cracks and voids in rock, the silica particles begin to settle out of solution and bond together. The suspended silica spheres diffract light to produce the vivid colors for which opal is known.

Precious opal forms in areas where silica gel has hardened very slowly over millions of years. The slower the rate of formation, the more orderly the arrangement of silica spheres within the opal. Valuable opals with vivid play-of-color form where silica spheres are all nearly uniform in size and packed into close-knit arrangements.

Common opal forms more rapidly in open cavities where silica spheres vary greatly in size. Without the uniformity of size and packing, common opal does not exhibit the play-of-color that makes precious opal so coveted. Black opal is a transparent to translucent opal with play-of-color against a black or other dark background.

While opal is not a rock, opal formations often contain partial replacements of the surrounding host rock. Wood opal forms when silica replaces organic material in wood, preserving its original structure. The opal may retain the cellular structure of the wood, making it easily identifiable.

Similarly, fossilized skeletal remains are sometimes partially or completely replaced by opal. Dinosaur bones, marine shells, and other fossils are occasionally opalized. Light passes through the opal that has replaced bone or shell to produce play-of-color, making such specimens both beautiful and highly valued.

So in summary, while opal may occasionally replace organic materials or rocks to produce gem-quality specimens, in its pure form it is neither a rock nor a true mineral. As an amorphous hydrated form of silica, opal stands alone as a unique mineraloid. Its mesmerizing play-of-color will likely continue to captivate gem aficionados for centuries to come.