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Coober Pedy opal is a precious gemstone mined in the South Australian desert town of Coober Pedy, characterized by its distinctive play-of-color and high silica content. The opal forms in the geological white claystone bands of the Great Artesian Basin, deposited during the Cretaceous period approximately 150 million years ago.
These opals exhibit exceptional qualities including vibrant spectral colors, high durability with a Mohs hardness of 5.5-6.5, and unique patterns of color play caused by the systematic arrangement of silica spheres. The stones often display brilliant combinations of red, blue, and green against a dark or light background matrix. The mining area spans approximately 4,954 square kilometers of arid terrain, where temperatures compel miners to work in underground dugouts.
The gemstones possess distinct classifications including black opal, crystal opal, and boulder opal, each formed under specific geological conditions within the ancient seabed sediments. These opals contain 6-10% water content and are composed of hydrated amorphous silica (SiO2·nH2O), creating their remarkable optical phenomena through diffraction of light.
Coober Pedy Opal is typically a natural gemstone.
Common names for Coober Pedy Opal include White Opal, Milk Opal, and Light Opal.
Coober Pedy opal, like most opals, has a relatively low hardness rating on the Mohs scale, typically ranging between 5.5 and 6.5. This makes it somewhat susceptible to scratching and requires careful handling and storage to avoid damage.
The refractive index of Coober Pedy opal usually falls within the range of 1.44 to 1.46, which contributes to its fascinating play of color.
Opals from Coober Pedy exhibit a vitreous to pearly luster, which enhances their visual appeal and is one of the reasons for their popularity in jewelry.
Opal, including Coober Pedy opal, generally does not have cleavage. This is a characteristic where the gem does not have natural planes along which it splits.
Coober Pedy opals have a sub-conchoidal to uneven fracture, which can make them challenging to cut and polish without causing damage.
The specific gravity of Coober Pedy opal ranges approximately from 1.98 to 2.25, which is typical for opals.
Opal is amorphous and does not exhibit double refraction. This is due to its lack of a crystalline structure.
Opals from Coober Pedy are renowned for their impressive play of color or “”fire,”” which is a form of dispersion. The colors can vary widely, showing a spectrum that can include greens, blues, reds, and oranges.
Being amorphous, opal does not have a crystal system. This means that its atomic structure is not regularly ordered.
Coober Pedy opals can display a wide array of colors, including white, gray, blue, green, and pink. The body color can greatly affect the value and desirability of the opal.
These opals can range from opaque to translucent, with transparency greatly influencing their aesthetic value and the way their color play is perceived.
Opal does not exhibit pleochroism due to its amorphous nature.
Coober Pedy opals can show fluorescence under ultraviolet light, typically displaying a white or greenish glow, which adds to their allure.
Despite their relative softness, opals have fair to good toughness, but they can be prone to chipping and cracking if exposed to rough conditions.
Opals are generally brittle, which means they can break or crack under stress.
Being isotropic, Coober Pedy opals do not have an optic sign.
Opals may exhibit a weak absorption spectrum with no significant peaks, largely due to their amorphous nature.
Coober Pedy opal is primarily made up of silicon dioxide (SiO2) and water (H2O), with water content typically ranging between 3% and 21%.
Some Coober Pedy opals may exhibit chatoyancy, but this is relatively rare compared to other gemstones like chrysoberyl.
Asterism is not commonly seen in opals from Coober Pedy.
The iridescence in Coober Pedy opals is one of their most sought-after features, characterized by vibrant and shifting color displays, which vary with the angle of light.
Opals, including those from Coober Pedy, are not magnetic.
Opals are generally non-conductive due to their amorphous and non-metallic nature.
Opals, including those mined in Coober Pedy, are not radioactive and pose no health risks in this regard.
Coober Pedy opals are cherished for their unique and striking play of color and their various other aesthetic qualities, making them a favorite among collectors and jewelry enthusiasts.
