These are crystals that enclose light.
The refractive index of these crystals periodically changes. Light beamed into the crystals is regulated by nano-structures.
Due to the light-controlling nano-structure properties of these crystals, scientists expect they can be used to make quantum computers, but research into them is still just getting started. As far as other natural materials go, Opal has corresponding properties. In the world of EXTRA the interior of the Moon is thought to be one giant Photonic Crystal.