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Lightning Ridge Opal is a rare and precious gemstone variety of hydrated silica, distinguished by its unique dark body tone and vibrant play-of-color, found exclusively in Lightning Ridge, New South Wales, Australia. This specific opal type forms in the weathered sedimentary rocks of the Griman Creek Formation, deposited during the Cretaceous period approximately 110 million years ago.
The stone exhibits distinctive spectral colors against its characteristic black to dark blue background, created by the diffraction of light through a precise arrangement of silica spheres. These opals form in vertical seams within ironstone boulders, often displaying brilliant red, blue, and green flashes against their dark body tone. Mining occurs through horizontal drives and vertical shafts in the ancient seabed deposits, with the most valuable specimens showing broad patterns of color, known as “”broad flash”” or “”sheet patterns.”” The gemstone’s formation requires specific geological conditions, including the presence of silica-rich solutions and seasonal wet-dry cycles, making Lightning Ridge Opal one of the world’s most sought-after precious gems.
Lightning Ridge Opal is typically a natural gemstone.
Common names for Lightning Ridge Opal include Black Opal, Dark Opal, and Matrix Opal.
Lightning Ridge Opal generally has a hardness of 5.5-6.5 on the Mohs scale. This relatively low hardness means that while it’s suitable for jewelry, it should be treated with care to avoid scratching.
The refractive index of Lightning Ridge Opal ranges from 1.37 to 1.52, which aids in its brilliant play of color.
This gemstone typically exhibits a subvitreous to waxy luster, contributing to its unique visual appeal.
Opal, including the type from Lightning Ridge, has no cleavage. This means it does not break along any specific flat plane.
The fracture is typically conchoidal, which can result in a smooth, curved surface when the stone is broken.
The specific gravity of Lightning Ridge Opal varies from about 1.98 to 2.25, which is relatively low and typical of opals.
Opal is amorphous and does not have a crystalline structure, thus it does not exhibit double refraction.
Lightning Ridge Opal is renowned for its exceptional play of color, also known as “”fire””. This is due to the diffraction of light passing through its internal structure.
Opal is amorphous, meaning it does not have a defined crystalline structure.
These opals can display an incredible range of colors, from vivid blues and greens to fiery reds and oranges, often all within the same stone.
Opals from Lightning Ridge can range from opaque to translucent. This variability can influence both their appearance and value.
Lightning Ridge Opal does not exhibit pleochroism due to its amorphous structure.
Some Lightning Ridge Opals may exhibit a weak to moderate fluorescence under ultraviolet light, typically showing green or white colors.
Opal is generally considered to have fair to poor toughness due to its brittleness and natural internal stresses.
The tenacity of opals is generally brittle, making them sensitive to being hit or dropped.
Being isotropic, Lightning Ridge Opal shows the same optical properties in all directions.
Lightning Ridge Opal does not have a characteristic absorption spectrum due to its amorphous nature.
Opal is a hydrated form of silica (SiO2·nH2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%.
Though rare, some opals from Lightning Ridge can exhibit chatoyancy when cut properly, displaying a cat’s eye effect.
Asterism, or the star effect, is extremely rare in opals but can occur in certain conditions with the right kind of included fibers and proper cutting.
The iridescence of Lightning Ridge Opal is arguably its most defining characteristic, with brilliant flashes of color that change with the angle of light.
Opals, including those from Lightning Ridge, are not magnetic.
Opal is generally a poor conductor of electricity.
Lightning Ridge Opal shows no significant levels of radioactivity.
