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Light opal is a translucent to semi-transparent variety of opal characterized by its pale, pastel background color and subtle play-of-color effect displaying predominantly white, yellow, and blue spectral hues. This gemstone exhibits a lower specific gravity ranging from 1.98 to 2.20 due to its higher water content compared to other opal varieties.
The internal structure consists of uniformly sized silica spheres arranged in a regular pattern, creating diffraction gratings that produce its distinctive optical phenomena. These spheres, typically measuring 150 to 300 nanometers in diameter, form a microstructure that determines the opal’s color play intensity. Light opals contain approximately 6-10% water by weight, with the molecular formula SiO2¬∑nH2O, where ‘n’ represents the variable water content.
Found primarily in Australia’s sedimentary deposits, particularly in the Lightning Ridge and Coober Pedy regions, light opals form under specific geological conditions requiring silica-rich solutions and proper pH levels. These gems rank 5.5-6.5 on the Mohs hardness scale and exhibit conchoidal fracture patterns.
Light opal is typically a natural gemstone.
Common names for Light Opal include White Opal, Milk Opal, and Silky Opal.
Light opal has a Mohs hardness rating of around 5.5 to 6.5, which makes it relatively soft compared to many other gemstones. This softness means it can be scratched by harder materials and requires careful handling and storage to avoid damage.
The refractive index of light opal ranges from approximately 1.37 to 1.52. This range can contribute to the stone’s unique play of color.
Light opal exhibits a subvitreous to waxy luster, which contributes to its beautiful and distinctive appearance.
Opal, including light opal, generally does not display any cleavage. This is a trait that makes it unique among many gemstones but also means it can be brittle and susceptible to cracking.
Light opal usually has a conchoidal to uneven fracture, typical for opals, which contributes to the stone’s unique shapes when broken.
The specific gravity of light opal typically ranges from about 1.98 to 2.25, which is relatively low compared to many other gemstones.
Opals, including light opal, are isotropic and therefore do not show double refraction. They have a single refractive index.
Light opal is well-known for its play of color, often referred to as “”fire.”” This optical phenomenon is due to the dispersion of light by silica spheres within the opal structure.
Opals, such as light opal, are amorphous rather than crystalline. This means they do not have a defined crystal system and are essentially composed of silica spheres in a tightly packed lattice.
Light opal displays a wide range of colors, often exhibiting many colors simultaneously. The background color tends to be white or very light, which enhances its play of color.
Light opals can range from opaque to translucent. The level of transparency affects the play of colors visible in the stone.
As an isotropic material, light opal does not exhibit pleochroism. The color play seen in opal is due to light interference from its internal structure rather than light absorption in different crystal directions.
Some light opals can show fluorescence under ultraviolet light, typically glowing a bright green or other colors depending on the specifics of the opal’s composition.
Light opal is generally considered to have fair to good toughness, but it can be prone to cracking and chipping due to its internal structure and water content.
Opal is generally brittle, which corresponds with its relatively low hardness and structural makeup.
Being isotropic, light opal has no optic sign.
Light opal may not show a distinctive absorption spectrum, unlike many crystalline gems. The color play is due to the diffraction of light.
Light opal is primarily made up of silicon dioxide (SiO2) and water. Its chemical formula can be represented as SiO2·nH2O, indicating the presence of water molecules within the silica structure.
While more common in other forms of opal, some light opals may exhibit a cat’s eye effect if they have aligned fibrous inclusions.
Star effects are rare in light opals due to their amorphous nature and lack of aligned inclusions needed to produce the effect.
The iridescence in light opal, often referred to as the play of color, is one of its most sought-after properties. This results from the diffraction of light by the microscopic silica spheres within the gemstone.
Light opal is not magnetic.
Opal, including light opal, is generally an insulator due to its composition of silica and water.
Light opal typically shows no radioactivity.
