Amorphous: Precious Opal and Cacholong

Precious opal is easily recognized for its play-of-color, or iridescence. This is caused by the unique structure of these specimens. Opal is made up of a lattice of SiO2 spheres. The spaces between these spheres are filled with water. This structure diffracts white light, creating an attractive iridescence. Not all opals have this property, however. Many are completely opaque, like cacholong, but are made of the same matrix of SiO2 and water as their colorful counterparts. This difference is due to tiny irregularities in the shape of the SiO2 spheres, which then scatter incoming light, rather than diffract it.

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