Opal

Opal is a special kind of mineral made mostly of silica and water. It is not a true mineral because it does not have a regular crystal structure. Instead, it is called a mineraloid. Opal forms when water carries tiny bits of silica and deposits them in cracks and spaces inside different kinds of rocks, such as sandstone and basalt.

The word "opal" comes from an old Sanskrit word meaning "jewel." There are two main types of opal: precious and common. Precious opal shows beautiful colors that change when you move it in the light. Common opal does not show these colorful patterns. Opals can be clear, cloudy, or even dark.

Precious opal

Precious opal shows beautiful colors that change depending on the angle you look at it. Even though it is not a true mineral, it has a special internal structure. Under a very strong microscope, precious opal is made of tiny balls of silica, arranged in a neat pattern. These tiny balls create the colorful patterns by bending and spreading the light that passes through them.

Because opal contains water, it can be sensitive to heat and can scratch easily. For use in jewelry, most opal is cut and polished into a smooth, curved shape called a cabochon. This shape allows you to see the colorful patterns best. Some thin pieces of opal are combined with other materials to make them stronger and more attractive. These combinations are called doublets and triplets.

Common opal

Besides the shiny gemstone opals that show beautiful colors, there are many other kinds of common opal. These include milk opal, which looks milky bluish or greenish; resin opal, which is honey-yellow and shiny like resin; wood opal, where wood turns into opal; menilite, which is brown or grey; hyalite, a clear glass-like opal; geyserite, formed around hot springs or geysers; and diatomaceous earth, made from tiny shell-like creatures. Common opal often looks hazy and milky inside the stone.

Varieties of common opal

"Girasol opal" is a special type of opal that shows a bluish glow or sheen that moves with the light. This effect comes from tiny particles inside the stone. You can find this kind of opal in places like Oregon and Mexico.

Another type of opal, called blue opal, is a blue-green stone found in Peru, Oregon, Idaho’s Owyhee region, and Nevada’s Virgin Valley.

Opal can also come from tiny plants called diatoms. When diatoms die, they can form layers at the bottom of lakes, bays, or oceans. Their cell walls are made of hydrated silicon dioxide. These layers create a material called diatomaceous earth, which is white and chalky and used in many ways.

History

Opal was a rare and valuable gem in ancient times. In Europe, royalty prized it highly. Before large deposits were found in Australia in the 1800s, the only known source of opal was in Červenica in Slovakia. Today, opal is the national gemstone of Australia.