Opal is a mineraloid, a material that lacks a crystal structure but has a regular composition of silica spheres of different sizes with varying spaces between them. If these spheres are similarly sized and regularly stacked, their arrangement diffracts light producing a play of color.

Arabian legend is that opals fell from the heavens in flashes of lightning. In Greek mythology, Gyges had an opal ring that could turn him invisible. He used it to kill the king of Lydia and then marry the queen.