Opal

Opal is a unique material known as a hydrated amorphous form of silica.

Because it lacks a formal crystalline structure, scientists classify it as a mineraloid rather than a true mineral. It is composed of silica (SiO2) and water (H2O). The water content usually stays between 6% and 10% by weight. However, this amount can range from as low as 3% to as high as 21%. Opals form at relatively low temperatures. They often settle into the fissures or cracks of various rocks. You might find them in sandstone, basalt, rhyolite, marl, or limonite.

There are two primary classes of opal: precious and common. Precious opal is famous for its play-of-color. This is a pseudo chromatic optical effect. It creates flashes of colored light when the stone is turned in white light. Common opal does not show this effect. Instead, common opal might show a hazy or milky sheen called opalescence. In gemology, opalescence is considered a form of adularescence. Depending on how it formed, opal can be transparent, translucent, or opaque. The background colors can be white, black, or almost any color in the visual spectrum. Black opal is the rarest variety, while white, gray, and green are more common.

The magic of precious opal comes from its microscopic internal structure.

At a microscopic scale, precious opal is made of silica nanospheres. These spheres are arranged in a hexagonal or cubic close-packed lattice. In the mid-1960s, J. V. Sanders showed how these spheres create color. The spheres are organized into closely packed planes. When white light passes through these planes, it undergoes diffraction and interference. This process follows Bragg's law of diffraction. The specific colors you see depend on the spacing between these planes. If the distance is about half the wavelength of visible light, the light diffracts. This creates the beautiful patterns we call play-of-color.

Because precious opal can be thin or sensitive, jewelers use different methods to create gems. Most opal is cut into a cabochon. This is a smooth, convex shape that maximizes the viewing angle of the colors. "Natural" opal consists entirely of precious opal. However, if the precious layer is too thin, makers create composite gems. An opal doublet uses a thin layer of precious opal backed by dark material. This backing, like ironstone or onyx, makes the colors pop. An opal triplet adds a third layer. This is a domed cap of clear quartz or plastic. The cap protects the stone and acts as a magnifier to show more color.

Opal has a long and storied history. In ancient times, it was extremely rare and valuable. In Europe, it was a gemstone prized by royalty. For example, the thirteenth-century crown of the Holy Roman Emperor featured an opal called "the Orphanus." This stone was described as white like snow with splashes of red wine. In 1584, Queen Elizabeth I received a full set of opal jewelry.

Most of these ancient stones likely came from mines in Slovakia. Those mines were worked until the late nineteenth century. Before the 1800s, Slovakia was the only known source of opal.

Today, Australia is the most important source of opal in the world. It is even the national gemstone of Australia.

Some data suggests Australian precious opal accounts for 95% to 97% of the global supply. The state of South Australia alone provides about 80% of the world's supply. The town of Coober Pedy is a major center for mining. In 1956, a massive stone called "Olympic Australis" was found there.

This gem is 270mm long and very valuable. Other important regions include Lightning Ridge in New South Wales and various sites in Ethiopia. While Ethiopia is a major producer, comparing its production to Australia's is difficult due to different reporting methods.

There are many surprising varieties of common opal. Some types are formed by biological processes. For example, wood opal forms when opal replaces organic material in wood.

Another type is formed by diatoms. Diatoms are a type of algae. When they die, their silica cell walls form layers on the ocean or lake floor. This creates diatomaceous earth, which is used in gardening and industrial filtering. You can also find hyalite opal, which is clear like glass. Other varieties include resin opal, which is honey-yellow, and geyserite, which forms around hot springs. Each variety shows how silica and water can interact with the natural world in different ways.