The other structures in the drawer are films and stacks — flat layers of order. An opal is order made solid: millions of silica spheres settled into a lattice like cannonballs in a crate. Each plane of spheres reflects one wavelength by Bragg's law, and because the lattice runs in three dimensions, whole domains flash different colors at once — the "play-of-color." No dye; the gem is the same dull glass throughout.
Drag left / right to tilt the stone — the viewing angle shortens the light's path and the flashes sweep toward blue. Drag up / down to grow or shrink the spheres and re-tune the whole palette.