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White Cliffs opal is a distinctive form of precious opal mined exclusively in White Cliffs, New South Wales, Australia, characterized by its unique seam structure and high silica content formed within Early Cretaceous sedimentary deposits.
The opal exhibits exceptional play-of-color, displaying vibrant spectral hues against a predominantly white to light gray matrix. Its formation occurred approximately 100 million years ago when silica-rich solutions percolated through sandstone layers, creating distinctive horizontal seams and vertical pipes. The geological conditions at White Cliffs produced opal with distinctive patterns, including “”pineapple”” formations and fossilized shells replaced by precious opal.
Mining operations at White Cliffs, established in 1889, employ specialized underground methods to extract opal from horizontal seams at depths ranging from 5 to 30 meters. The region’s harsh desert climate and unique geological features contribute to the opal’s distinct characteristics, including higher structural stability compared to other Australian opals.
White Cliffs is typically a natural gemstone.
Common names for White Cliffs opal include milk opal, white milk opal, and porcelain opal.
White Cliffs Opal typically has a hardness of 5.5 to 6.5 on the Mohs scale, making it relatively softer than many other gemstones. This level of hardness indicates that while opal is durable enough for use in jewelry, it is susceptible to scratches and abrasions and should be worn and stored carefully.
The refractive index of White Cliffs Opal ranges from about 1.37 to 1.52. This contributes to the unique play of color that opals are known for, as light bends when entering the stone, separating into spectral colors.
White Cliffs Opal exhibits a lustrous sheen, generally described as vitreous to pearly. The luster of an opal is one of its most attractive features, contributing significantly to its visual appeal.
Opal does not have any natural planes of cleavage. This means that it does not split along any definite plane when subjected to stress, which is beneficial for its durability in various settings.
The fracture of White Cliffs Opal is typically conchoidal, creating a smooth, curved surface when the stone is broken. This is a common characteristic in minerals that lack cleavage.
The specific gravity of White Cliffs Opal is typically in the range of 1.98 to 2.25. This measures the density of the opal in comparison to water.
Opal is an amorphous gemstone and does not have a crystalline structure; therefore, it does not exhibit double refraction. It is considered isotropic.
One of the most notable properties of White Cliffs Opal is its dispersion or “”fire,”” which is the ability to disperse light into multiple colors. This fire is caused by the diffraction of light through the silica spheres within the structure of the opal.
Opal is amorphous, meaning it does not have a crystalline structure. This lack of a regular atomic structure contributes to its unique light interactions.
White Cliffs Opal can exhibit a range of background colors, but they are predominantly known for their white to light body tone. The color play can include a full spectrum of colors depending on the conditions under which the opal formed.
Opals from White Cliffs can range from opaque to translucent. The level of transparency affects the play of color and the overall aesthetic quality of the stone.
Pleochroism is not typically observed in opal due to its amorphous nature. The color play in opal is due to light diffraction, not pleochroism.
White Cliffs Opal may exhibit a weak to moderate fluorescence under ultraviolet light, often showing green or white hues. The presence of fluorescence can vary from one opal to another.
Opal is generally considered to have fair to good toughness, but it can be prone to cracking or “”crazing”” under extreme temperature changes or dehydration.
Opal is relatively brittle and can be damaged if subjected to harsh impacts or pressure. Care should be taken when setting and wearing opal jewelry.
Being isotropic, White Cliffs Opal shows the same optical properties in all directions and does not have a specific optic sign.
Opal generally does not produce a characteristic absorption spectrum due to its amorphous nature and the unique way it interacts with light.
Opal is a hydrated form of silica (SiO₂·nH₂O). Its water content can range from 3 to 21% by weight, but is typically between 6 and 10%.
While less common than in some other gemstones, some opals like White Cliffs Opal can display a cat’s eye effect if they have aligned fibrous inclusions.
Asterism, or the star effect, is not typically seen in opals. This effect is more commonly associated with corundum (sapphires and rubies).
Iridescence is a fundamental characteristic of White Cliffs Opal, often seen as a play of colors or a “”fire”” that changes with the angle of light.
Opal is not magnetic and does not respond to a magnet.
As a non-metallic mineral, opal has poor electrical conductivity.
Opal is not radioactive and poses no health hazards in this regard.
