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Pearl opal is a rare variety of opal exhibiting a distinctive pearlescent sheen combined with the characteristic play-of-color found in precious opals. This hydrated amorphous form of silica contains between 6-10% water content and displays a unique structural composition of microscopic silica spheres arranged in a regular pattern.
The gemstone manifests an exceptional optical phenomenon known as adularescence, similar to moonstone, while simultaneously producing spectral color displays through diffraction of light. Its internal structure consists of both ordered arrays of silica spheres responsible for opalescence and oriented crystal platelets that create the pearl-like lustre. The most valuable specimens originate from Australian deposits, particularly Lightning Ridge, where they form in marine sedimentary environments under specific geological conditions.
These opals typically display a white to cream base color with rainbow iridescence overlaid by a silvery-white sheen. Their Mohs hardness ranges from 5.5 to 6.5, making them relatively soft compared to other gemstones. Mineralogists classify pearl opals within the tectosilicate group, distinguished by their unique combination of optical effects.
Pearl opal is typically a natural gemstone.
Common names for Pearl opal include Hyalite, Water Opal, and Muller’s Glass.
Pearl opal has a hardness of approximately 5.5 to 6.5 on the Mohs scale, making it relatively soft compared to many other gemstones.
The refractive index of pearl opal generally ranges from 1.44 to 1.46, which is relatively low and contributes to its subtle light play.
Pearl opal exhibits a waxy to pearly luster, which is characteristic of opal varieties. This luster gives it a soft, appealing glow that is highly valued.
Opal, including pearl opal, does not have any natural planes of cleavage; it is typically amorphous, meaning it lacks a defined crystalline structure.
Pearl opal typically exhibits a conchoidal fracture, which is a curved breakage surface resembling the inner surface of a shell.
The specific gravity of pearl opal ranges approximately from 2.1 to 2.2, which is relatively light compared to many other gemstones.
Being amorphous, pearl opal does not exhibit double refraction, as it lacks a crystalline structure.
Pearl opal can show a degree of dispersion or fire, but it is generally less pronounced than in other types of opals like precious opal.
Pearl opal is amorphous and does not have a crystal system.
The color of pearl opal can vary widely, including white, cream, and gray, often with an iridescent play of colors depending on the angle of light.
Pearl opal can range from opaque to semi-translucent. The level of transparency often affects the gemstone’s value and aesthetic appeal.
Pleochroism is not typically observed in pearl opal due to its amorphous structure.
Some pearl opals may show fluorescence under UV light, typically glowing in green or white tones.
Pearl opal has fair to poor toughness due to its inherent brittleness and sensitivity to pressure and impact.
The tenacity of pearl opal is generally brittle, making it susceptible to chipping and cracking if not handled with care.
Being amorphous, pearl opal does not have an optic sign.
Pearl opal does not typically show a distinctive absorption spectrum due to its amorphous nature and material properties.
Pearl opal is primarily composed of hydrated silicon dioxide (SiO₂·nH₂O).
Chatoyancy in pearl opal is rare but can occur if the stone has aligned fibrous inclusions.
Asterism or a star effect is not commonly observed in pearl opal.
The iridescence in pearl opal, similar to other opals, is due to the diffraction of light by silica spheres within the structure, creating a play of color.
Pearl opal is not magnetic.
Pearl opal has low electrical conductivity.
Pearl opal is not radioactive.
Pearl opal, with its subtle beauty and play of color, is a unique and cherished gemstone, often used in jewelry that benefits from its distinctive optical properties and aesthetic appeal.
