Opal is one of geology’s strangest mineral materials: solid stone made from silica and water, but without the orderly crystal structure found in minerals like quartz.

Precious opal is famous for play-of-color: the shifting rainbow flashes produced when tightly packed silica spheres split light into spectral colors.

Common opal (also called potch) forms from the same material, but its internal structure is less uniform, so it usually does not produce those blazing rainbow flashes.

The soft glow, waxy surface, and milky translucence of common opal are called opalescence: a pearly optical effect where light scatters inside the stone, giving it a cloudy, glowing, almost suspended look.

Opal forms when silica-rich water seeps into cracks, cavities, volcanic rocks, or sedimentary layers, then gradually dries and hardens. That process can take thousands to millions of years, depending on the conditions. Because it lacks a regular crystal structure, mineralogists often call it a mineraloid rather than a true mineral.