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Common opal is a hydrated amorphous form of silica (SiO2·nH2O) characterized by its non-iridescent, translucent to opaque appearance and variable coloration ranging from white to red, yellow, blue, or green. The mineral forms through low-temperature hydrothermal processes where silica-rich solutions deposit in rock cavities, fractures, and sedimentary layers.
This mineraloid exhibits a vitreous to waxy luster, conchoidal fracture, and a hardness of 5.5-6 on the Mohs scale. Its internal structure consists of densely packed spheres of silica measuring 150-300 nanometers in diameter, arranged in a non-periodic pattern that distinguishes it from precious opal’s ordered arrangement. The mineral contains 6-10% water by weight, with the exact percentage affecting its physical properties and stability.
Common opal occurs globally in volcanic rocks, particularly within weathered basalts, rhyolites, and in sedimentary environments. Notable deposits exist in Australia, Mexico, Brazil, and the United States, where it often forms through the silicification of wood, bone, and other organic materials.
Common Opal is typically a natural gemstone.
Common names for Common Opal include potch, milk opal, and jelly opal.
Common opal has a hardness of 5.5 to 6.5 on the Mohs scale, making it relatively soft compared to many other gemstones. This hardness level suggests that while it can be scratched by harder materials, it is suitable for use in various types of jewelry.
The refractive index of common opal ranges from 1.44 to 1.46. This property measures how much light is bent, or refracted, when entering the stone.
Common opal is known for its waxy to dull luster, which gives it a unique soft glow different from the brilliant sparkle of crystalline gems.
Opal exhibits no cleavage, which means it does not break along defined planes. This characteristic makes it less predictable to cut.
Common opal typically fractures in a conchoidal manner, similar to glass, producing smooth, curved surfaces.
The specific gravity of common opal ranges approximately from 1.98 to 2.25. This measurement indicates the density of the gem compared to water.
Common opal is isotropic and shows no birefringence due to its amorphous nature. This means that light does not split into two rays when passing through the gem.
Common opal generally has low to no dispersion or fire, unlike its counterpart, precious opal, which is famed for its colorful play-of-color.
Opal is amorphous, meaning it does not have a crystalline structure. This lack of internal structure contributes to its unique light interactions and physical properties.
Common opal can come in a variety of colors including white, gray, blue, green, and pink. However, it lacks the play-of-color seen in precious opals.
Common opal can range from transparent to opaque. The level of transparency affects the gemstone’s appearance and its use in jewelry.
This gemstone exhibits no pleochroism due to its isotropic, amorphous nature.
Some common opals may show fluorescence under UV light, typically glowing green or white. However, this property can vary depending on the opal’s origin and specific composition.
Despite its fair to good toughness, common opal is relatively fragile due to its high water content and must be treated with care to avoid drying out and cracking.
Common opal is generally brittle. This means that care must be taken during setting, wearing, and storing to avoid damage.
Being isotropic, common opal shows a single refractive index and therefore has no optic sign.
Common opal does not typically display a characteristic absorption spectrum due to its amorphous structure.
Common opal is a form of silica (SiO‚ÇÇ) and contains 6-10% water. This composition can vary slightly with different inclusions and trace elements.
While less common than in some other gemstones, certain opals may exhibit a cat’s eye effect if they possess aligned fibrous inclusions.
Asterism or a star effect is very rare in opals and not typically associated with common opal.
Some common opals may display a form of iridescence, but not the same as the play-of-color seen in precious opals. This is usually due to the diffraction of light off of microscopic silica spheres within the stone.
Common opal is not magnetic.
Opal is generally an insulator and does not conduct electricity well.
Common opal is not radioactive and poses no health risks in this regard.
