Opal is a member of thequartzfamily. Opal is considered an acrystalline or noncrystallinequartz, meaning that instead of being made of tiny crystals, opal is made from tiny spheres or balls of silica and water content of up to 21%. Since opal has no crystal structure, it is not classified as a mineral but a mineraloid.
When most people think of opal, they normally think of glittering rare precious opal and it's dazzling rainbow of colors. This occurs when the balls of silica are all the same size and the water content is stable.
What most people do not realize that common or potch opal is very common and everywhere around them. Opal forms when free silica in water settles out without crystallizing in a cool environment, like cooler hot springs. Many of our favorite gemstones we enjoy in casual and fine jewelry are forms of common opal.
Opal artifacts, several thousands of years old have been discovered in East Africa and America. The Romans were known to use opal back to 250 B.C. coming from mines in eastern Europe.
Opal has a hardness of a 6 on the Moh's scale of hardness and makes a great choice for earrings and necklaces. Opal is the birthstone for October.