Amorphous

Amorphous gem materials include opal, obsidian, jet, and natural glass (tektites). Because they have no crystallographic axes, amorphous gems are always isotropic — they do not split light into two rays and show no birefringence. Opal's play-of-color arises not from crystal structure but from the diffraction of light by regularly stacked silica spheres, a structure sometimes called 'paracrystalline.' Amorphous materials tend to be softer and less chemically stable over geological time compared to crystalline equivalents.