Opal

Opal is a very unique gem that is not actually a crystal! In order for a material to be a crystal its chemical structure most be uniform. When silica has a uniform structure its a variety of Quartz which is an electric conductor, when its not its an amorphous material like glass which is an insulator. Opal is essentially glass spheres with a slightly different structure that slowly compact on each other over time until it forms into one stone

The notorious play of color is caused by the light passing through all those little silica spheres and creating a bunch of micro prisms. Opal also comes in hydrophane and non hydrophane meaning some opals are porous and absorb water (hydrophane) and some opals do not (non hydrophane). Opals tend to be sensitive stones and the best practice is to treat them all the same regardless, they should always maintain some level of moisture, allowing opals to become too dry or too saturated can destroy the stone

Hardness: 5 – 6.5
Composition: SiO₂H₂O