How is Opal Made?

Opals are known for their rainbow ripples of color and light.

While the largest reserve of opals are in Australia, the stones have been unearthed worldwide. The vibrant, firecracker of colors inherent in the gems have stirred imaginations in numerous cultures. From a scientific point of view, the opal fascinates as it is an example of how the friction between water and light can create luminous stones.

Identification

Unlike minerals, mineraloids do not have a crystal structure.

Opals are distinguished by their iridescent, rainbow-like fluttering of colors. Technically, opals are mineraloids, which are similar to minerals. Minerals are solids that form naturally in the earth and have a crystalline composition, otherwise known as a specific arrangement of particles. Mineraloids, such as opals, amber and obsidian, lack crystallinity but are still contain chemical compositions. Specifically, opals are a mix of hydrous silicon dioxide and between three and 21 percent water. Silica is a transparent, colorless crystal found in glass and ceramic. Opals are formed when water sluices downward through the earth, absorbing sandstone along the way. The sandstone in turn saturates the water with silica, which then settles between breaks in the earth. Water evaporates, leaving a silica residue behind. As the process repeats itself, opal is slowly formed.

Effects

The way light moves through opal's chemical composition is what makes the colors so intense.

The hydrated chains of silica and oxygen create microscopic spheres. According to the U.S. Geological Survey, when light cuts through the spheres, it causes the variegated prisms of color. This disproves the assumption that the water alone stimulates the coloration, a fact that wasn't discovered until Australian scientists unlocked the secret in the 1960s.

Geography

The Great Artesian Basin is a huge source of opals.

While roughly 5 percent of opal fields exist in Mexico, Brazil, Ethiopia, Mali and even America in the states of Idaho and Nevada, 95 percent of the stones are located in and around the Great Artesian Basin, an area that stretches from Queensland to the northeastern portion of South Australia and the northwestern region of New South Wales; roughly a fifth of the country of Australia. The area is opal-rich because it was submerged in water during the age of the dinosaurs.

Processing

Opal cutters use sandpaper and wet leather wheels to refine the opals shape.

Once opals are unearthed, they are polished and cut into oval or round cabochons, a French word that defines gems cut in convex shape, (a shape that curves outward like a circle's circumference). The opal cutter then uses a diamond cutting wheel to shave off impurities and to roughly shape the stone. Sandpaper is used to finalize the shape before the opal is perfected with a wet leather wheel.

HIstory/Legend

Aborigines believed that rainbows were the source of the opal's color.

Gemselect.com quotes the words of the famous Roman author, Pliny, who praised the opal as being a stone that "combines the fine sparkle of almandine, the shining purple of amethyst, the golden yellow of topaz and the deep blue of sapphire." The Greeks cherished opals for it was said that they were the elated tears that Zeus wept after he bested the Titans. In Arabic culture, opals were supposedly born from lightning. Aborigines, meanwhile, worshiped the stone because they believed that the creator descended from the heavens on a rainbow and left rainbow-colored stones with every earthbound step he took.

Theories/Speculation

According to gemstone.org, one speculation is that the opal got its name from the Sanskrit word "upola," which means "valuable stone." Another hypothesis is that it stems from the Greek word for "color change," "opallios," or from the Roman word "opalus" which translates to "stone from several elements."

Types

Aztecs and Mayans used the fire opal in many rituals.

Opals come in many different types, the rarest and most costly being the black opal. Its opaque body is solid and sometimes has slivers of peacock-blue color. Roughly 99.9 percent of black opals are mined in Mintabie and Andamooka, Australia. Light opal deposits are centered in South Australia and are identified by the shades of bright blue scintillating across the moonstone-white surface. Fire opals are commonly found in Mexico's highlands, along the edge of dormant volcanoes. Mayans and Aztecs referred to the fiery-red stone as the "stone of the bird of paradise" and used it in many rituals. With a respectable hardness that averages between 6 and 6.5 out of 10 on the Mohs scale of mineral hardness, it is considered the official stone of Mexico.

References

• Gemstone: OOpal Lore
• Opals Down Under: Australian Opals
• Costellos: Opal Types
• Opals Down Under: Great Artesian Basin