What Is Opal? Also Not Technically a Mineral

Opal doesn't meet the strict definition of a mineral either, for a similar reason to obsidian: it lacks an ordered crystal structure. Instead, it's made of tiny silica spheres stacked together with water trapped between them. When those spheres happen to be uniform in size and stacked in a regular pattern, they diffract light into flashes of color — known as play-of-color. Irregular stacking produces "common opal," with no flash at all.

Opal genuinely contains water within its structure, often 3–21% by weight, which means it can crack ("craze") if it dries out too quickly under extreme heat or low humidity — worth knowing if you keep opal jewelry somewhere very dry. Australia produces the large majority of the world's precious opal; Ethiopia and Mexico (source of fire opal) are other significant origins.