About Australian Opal

Opal Composition

Opal is composed of hydrated amorphous silica with the chemical formula S:O2.nH2O. It has a hardness of between 5.5 and 6.5 on the Moh’s scale and a specific gravity of 1.9 to 2.3. Most opal is more than 60 million years old and dates back to the Cretaceous period when dinosaurs roamed the earth. It is found near the earth’s surface in areas where ancient geothermal hot springs once flowed. The minerals bubbled up from beneath the surface of the earth and slowly, over centuries, lined the walls of cracks, vents, and underground cavities in the bedrock forming opal. Most opal is found where geothermal hot springs dried up during seasonal periods of rainfall and then extended dry periods.

Opal Mining

Almost 95% of all opal comes from Australian opal mines in the remote outback deserts. The remaining 5% is mined in Mexico, Brazil and in the United States in Idaho and Nevada. Opal has also recently been discovered in parts of Africa. Opal was first discovered in Australia in 1849 and prospecting began in 1890. White Cliffs, Lightning Ridge, Andamooka, and Coober Pedy are some of the legendary fields of Australian opal mining. The most famous area is Lightning Ridge where “black opal” is mainly found.

Opal Grades

High quality black opal is more valuable than diamond, costing up to $20,000 per carat. Precious opal is opal which exhibits the phenomenon known as play-of-color, produced by the diffraction of white light through a micro structure of orderly arrayed silica spheres to produce changing spectral hues. These brilliant areas of color can be seen within the stone and change in hue and size as the stone is moved. The interplay of colors gives opal its unique visual appeal, and makes it one of the most sought after gemstones in the world. Crystal opal, which follows black opal in value, is nearly transparent with a deep play of colors. The least expensive is white or milky opal which shows more diffused colors in a white base. No matter what the grade, each piece of opal truly is a unique and one-of-a-kind masterpiece created by nature.