August 8, 2022

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What Distinguishes Australian Opals From Other Opals?

While opals have been around for hundreds of thousands of years, they are still considered one of the rarest gems. The Australian opal is one type that stands out from other types due to its unique formation and color variations. This article will explore the differences between an Australian crystal opal and different types of precious stones.

How and Where Are These Opals Formed?

Most black opals are formed in the horizontal zones of sandstone beds. The Black Opal is found at Lightning Ridge and Mintabee, with the primary source near Coonaweelah, where the rocks are about 100 million years old. The material is a light to dark grey potch, a soft sedimentary rock transformed into an amorphous material by millions of years of pressure and heat. At Lightning Ridge and Mintabee, most black opals formed in the horizontal zones of sandstone beds. In Queensland, most black opals formed in vertical cavities facing north or east, such as fissures and joints.

The Australian black opal is not as common as its other colors but just as beautiful. The Australian opal mines have produced these beautiful gems for over 100 years.

The Main Colours of Australian Opals 

Australian opals are known for their beautiful colors and patterns. But what makes an Australian opal so different from other types of opals?

The main colors of Australian opals are white, black, crystal and boulder, red, pink, orange, and green, which makes them even more desirable among buyers who want something unique.

The color of an Australian opal depends on the type of stone used to make it and how the stone was treated before being cut into an oval shape to create a polished cabochon.

  • The most common treatment involves heating the stone with natural gas or oil until it reaches a temperature between 500 degrees Celsius and 1000 degrees Celsius (932 degrees Fahrenheit). This process causes impurities within the stone to separate from its matrix while leaving behind iron sulfide as a residue on its surface.
  • White Australian opal is created by heating iron-rich clay found in Lightning Ridge, which contains mostly silicon dioxide but also traces amounts of metals like copper, nickel, or chromium oxides.

The Magnificence of Black Opals

The most valuable of all opals is the black opal. Black opal has an iridescent display of colors against a dark background, which makes it very rare and valuable.

The mining process to find these valuable gems can be challenging, as they are found in Australia at depths up to 100 feet below ground level. The miners must dig through solid rock with heavy machinery and then use hand tools to break apart the rock to access the black opal veins inside.

Several types of natural stones are found in Australia; however, only 1% of these stones become precious black opals!

Boulder Opal Requires Careful Cutting

The term boulder opal is used for a rock matrix that contains a significant amount of precious opal. The host rock may be ironstone, sandstone, or shale. Boulder opal requires careful cutting to retain the natural form of the host rock.

Boulder opals are usually cut into cabochons but can be faceted for rings and other jewelry.

For the Perfect Ornamental Pieces

Black opals are lighter in color than others, so it’s usually fashioned into a ring or pendant rather than set as a solitaire stone on a necklace. The crystal opal and white opals can be considered “imitation” or synthetic stones. These aren’t genuine opals but have instead been treated with chemicals to appear similar to natural stones.

A beautiful Australian-style necklace made entirely of black opal cabochons looks mesmerizing with just about anything. 

In summary, many qualities make an opal an excellent investment. This includes the fact that it is considered one of the world’s most precious stones due to its unique properties and rarity.