What causes the play of color in opal?
Opal is a name for a number of minerals that are silicon dioxides with variable amounts of water. It is more crystalline than glass and less crystalline than quartz and it’s high temperature and pressure phases. Opal can grow in a number of different ways. It can rarely precipitate out of vapor in volcanic fumarole deposits. It is the major silicon mineral created by biological processes. Opal most commonly is precipitated from an aqueous solution when the ability of water to dissolve silicon is exceeded. Silicon can dissolve into water from minerals at the surface and be carried down to the groundwater. Precious opal gemstones are created when the opal forms small spheres of the mineral in a gel and settles to form a natural diffraction grating. Common opal or potch consists of particles that are either of varying size or not spheres. Etching of precious opal indicates spheres that consist of a small core surrounded by concentric rings of opal. The Australian opal is believed to have
