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Queensland opal is a precious gemstone formed from hydrated silica deposits found predominantly in Australia’s Great Artesian Basin, characterized by its distinctive play-of-color and exceptional brightness across the visible spectrum. The stone exhibits a unique internal structure of uniformly-sized silica spheres arranged in a regular grid pattern, creating diffraction gratings that produce spectral colors through the Bragg diffraction phenomenon.
These opals formed during the Cretaceous Period when silica-rich solutions percolated through sandstone and mudstone layers, depositing in cavities and replacing organic materials. Queensland specimens display remarkable patterns including ‘boulder opal’ where the precious opal forms in ironstone matrix, and ‘yowah nuts’ – distinctive spherical concretions containing opal cores. The finest examples exhibit vivid spectral colors against either a dark or light background, with red flash patterns being particularly valued in the international gemstone market.
Queensland opal is typically a natural gemstone.
Common names for Queensland opal include boulder opal, matrix opal, and pipe opal.
Queensland opal typically has a hardness of 5.5 to 6.5 on the Mohs scale, making it relatively softer compared to many other gemstones. This level of hardness necessitates careful handling to avoid scratches or abrasions.
The refractive index of Queensland opal ranges from approximately 1.37 to 1.52. This property helps to give the opal its unique play of color.
Queensland opal exhibits a lustrous shine that can range from vitreous (glass-like) to subvitreous and waxy.
Opal, including Queensland opal, typically does not have cleavage. This refers to the way a mineral breaks along specific planes.
This type of opal often has a conchoidal (shell-like) fracture, which is a common characteristic in amorphous and glassy minerals.
The specific gravity of Queensland opal varies between approximately 1.98 and 2.25, depending on the amount of water content and inclusions within the stone.
Opal, including Queensland opal, is amorphous and does not have a crystalline structure, thus it does not exhibit birefringence or double refraction.
Queensland opal is renowned for its spectacular play of color, often referred to as “”fire.”” This fire is the result of the diffraction of light passing through the silica spheres within the structure of the opal.
Opal is amorphous, meaning it does not have a crystalline structure. This differentiates it from crystalline gemstones.
Queensland opal can display a wide range of colors, including white, black, and various shades of red, green, and blue. The background color can greatly influence the perceived intensity and type of color play.
The transparency of Queensland opal can vary significantly from opaque to translucent and occasionally transparent.
As an amorphous gemstone, Queensland opal does not exhibit pleochroism.
Some Queensland opals may show fluorescence under ultraviolet light, typically displaying a greenish or bluish glow.
Opal has fair to good toughness, but its water content and natural inclusions can make it prone to cracking or “”crazing”” under extreme temperature changes or dehydration.
Queensland opal is generally brittle, which is typical for opals due to their amorphous nature and internal structure.
Being amorphous, Queensland opal does not exhibit an optic sign.
Queensland opal does not typically produce a distinctive absorption spectrum, unlike many other gemstones which have a crystalline structure.
Opal is primarily hydrated silica (SiO2·nH2O). The water content can range from 3% to 21% by weight.
While not as common as in other gemstones, some Queensland opals can exhibit chatoyancy when cut properly, especially when they contain aligned inclusions.
Asterism or the star effect is rare in opals but can occur if the stone’s inclusions are appropriately aligned.
The iridescence in Queensland opal, often referred to as the play of color, is one of its most sought-after characteristics, resulting from the interference and diffraction of light.
Opal, including Queensland opal, is not magnetic.
As a non-metallic mineral, Queensland opal is a poor conductor of electricity.
Queensland opal is not radioactive.
