How Are Opals Formed?

Drying minerals

Opals are formed when a mineraloid gel mixed with water begins to dry. A mineraloid gel is a mixture of minerals and chemicals that does not contain any crystals. The gel will not crystallize when it dries; instead it will harden and create another type of stone. For opals, as the mixture dries it leaves behind a rigid structure of silica that will be able to refract and diffuse light. Opals are considered precious gems when the gel has less than 10 percent of water left in the structure. This means that the gel has hardened enough and will not change shape any more.

Colors

The most common opals are a white, milky color. Other common opals are a blue and green color. The most popular opals are red and orange opals, called fire opals. Other opal colors include gray, black, brown, yellow and pink. Opals can come in any color of the rainbow. The color of the opal is determined by the space between the microscopic spheres that make up the structure of the opal. The spaces allow for the diffusion of light. The longer the wave of light, the more red the opal will look and the shorter the wave of light, the more violet the opal will look.

Where to find opals

Opals are mostly mined and found in Australia. South Australia leads the world in producing opal. Opal is the country's largest export.