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Dark opal is a rare form of precious opal exhibiting a dark body tone, predominantly black, gray, or blue, against which vibrant play-of-color displays in spectral hues. This gemological phenomenon occurs in silica deposits where spheres of amorphous silicon dioxide create a three-dimensional diffraction grating.
The most valuable dark opals originate from Lightning Ridge, New South Wales, Australia, where they formed in ancient sedimentary rocks during the Cretaceous period. These specimens display distinct optical characteristics, including directional color patterns known as ‘rolling flash’ and ‘broad flash,’ caused by the precise arrangement of silica spheres sized between 150-400 nanometers.
Dark opals contain 6-10% water content within their chemical structure, distinguished by their refractive index of 1.37-1.47 and specific gravity ranging from 1.98-2.20. The darkness of their body tone results from trace elements and carbon inclusions, which enhance the contrast and intensity of their prismatic color display.
Dark Opal is typically a natural gemstone.
Common names for Dark Opal include Black Opal, Boulder Opal, and Chocolate Opal.
Dark opal, like other types of opal, has a relatively low hardness, typically ranging between 5.5 and 6.5 on the Mohs scale. This makes it somewhat vulnerable to scratching and requires careful handling and storage.
The refractive index of dark opal ranges from about 1.37 to 1.52. This variation depends on the specific structure and composition of the opal.
Dark opal exhibits a subvitreous to waxy luster, which contributes to its unique visual appeal, enhancing the play of color that opals are known for.
Opal, including dark opal, generally does not exhibit cleavage. This is because opal is amorphous, meaning it lacks a crystalline structure.
Dark opals typically have a conchoidal fracture, which is a curved break similar to that seen in glass.
The specific gravity of dark opal can vary from about 1.98 to 2.25, which is relatively heavier compared to some other gemstones due to its water content.
Opal is isotropic and does not show double refraction due to its amorphous nature.
While dark opal may exhibit less fire than lighter opals, it can still display a broad range of colors. The dispersion, or fire, can vary widely and is one of the key attractions of opal.
Dark opal does not have a crystal system because it is amorphous (non-crystalline).
The body color of dark opal can range from dark gray to black, and it generally exhibits a vibrant play of color. The darker body color in dark opals enhances the visibility of the color play.
Dark opals range from opaque to translucent. This range affects the visibility of the color play within the stone.
Opal, including dark opal, displays no pleochroism due to its isotropic nature.
Dark opals may show weak to moderate fluorescence under UV light, with colors typically ranging from green to white.
Opal is generally considered to have fair to good toughness, but it can be brittle and sensitive to changes in temperature and humidity.
Opals are generally brittle, and dark opals are no exception. They can be prone to cracking and chipping if handled roughly.
Being isotropic, dark opal has no optic sign.
Dark opals generally do not exhibit a characteristic absorption spectrum due to their amorphous structure and varied composition.
Dark opal is a form of silica (SiO₂·nH₂O) and contains a significant amount of water, typically between 3% and 21%.
While typically not common in dark opals, some stones may exhibit a cat’s eye effect if they have aligned fibrous inclusions.
Asterism or the star effect is rarely seen in opals, including dark opals.
The color play (“”opalescence””) seen in dark opals is a form of iridescence. This is caused by the diffraction of light passing through the microscopic silica spheres within the structure of the opal.
Dark opal is non-magnetic.
Opals, including dark opals, are generally poor conductors of electricity.
Under normal circumstances, dark opal does not exhibit radioactivity.
