Play-of-Color

1950s J.E. Caldwell Platinum, GIA-Certified opal, sapphire & diamond ring
Photo courtesy of Scott Papper, GIA GG, AJP, CM (NAJA), Global Gemology & Appraisals
Some opals, but not all, possess a phenomenal property known as play-of-color.

Opals are made up of submicroscopic spheres stacked in a grid-like pattern.  As light passes between the spheres, the lightwaves bend, breaking up into different spectral colors.