When spheres of hydrated silica align in a regular array, they diffract light and produce spectral colour. The effect is called "play of colour." It requires spheres close in diameter to the wavelength of visible light.

Australia accounts for roughly 95 per cent of global production. Coober Pedy, known as the opal capital of the world, is so arid that the town was built underground to escape the surface heat.

Because it contains water, it can crack in dry conditions. Water trapped inside stone. A mineral, but not entirely solid.