October — Opal

Opal is not a crystal. It is silica spheres packed in a lattice with water still trapped between them — up to a fifth of the stone by weight. The colours are not pigment; they are light diffracting off the spacing of those spheres, which is why the fire moves when you move.

Because of the water it is softer and more temperamental than most stones. That is the trade for what it does with light.