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Precious opal is a hydrated amorphous form of silica (SiO2·nH2O) displaying play-of-color, a unique optical phenomenon caused by the diffraction of light through a regular array of silica microspheres. These microspheres, ranging from 150 to 300 nanometers in diameter, create a three-dimensional structure that breaks white light into spectral colors.
The gem forms in fissures of rocks when silica-rich water penetrates into cavities and gradually solidifies at relatively low temperatures. Australia produces approximately 95% of the world’s precious opal, with significant deposits in Lightning Ridge, Coober Pedy, and Andamooka. The finest specimens exhibit vivid spectral colors against dark body tones, with red flash being the most valued.
Precious opal’s internal structure consists of regularly packed spheres of silica in a hexagonal or cubic close-packed lattice, distinguishing it from common opal which lacks this ordered arrangement. The gem contains 6-10% water by weight, making it susceptible to crazing under rapid temperature changes or dehydration.
Precious opal is typically a natural gemstone.
Common names for Precious opal include fire opal, black opal, boulder opal, crystal opal, white opal, and milk opal.
Precious opal has a hardness rating of 5.5 to 6.5 on the Mohs scale, making it relatively softer compared to many other gemstones. This means it’s somewhat susceptible to scratches and needs careful handling.
The refractive index of precious opal ranges from 1.37 to 1.52. This variability contributes to its unique play of color.
Precious opal exhibits a subvitreous to waxy luster, which enhances its visual appeal and is particularly noticeable when the stone is well-polished.
Opal generally does not display any cleavage, which is the tendency of a mineral to break along flat planes.
It usually has a conchoidal (shell-like) fracture, which can make cutting and shaping the gem a delicate process.
The specific gravity of precious opal varies between 1.98 and 2.25, which is relatively low, indicating a lighter gemstone.
Precious opal is isotropic and shows no double refraction. This means that light does not split into two separate rays when passing through the gem.
Opal is well-known for its remarkable dispersion or “”fire,”” which refers to its ability to disperse light into multiple colors. This property is especially prominent in precious opal.
Opal has an amorphous crystal system, meaning it lacks a defined crystalline structure. This amorphous nature is responsible for its distinctive play of color.
Precious opal can display a wide range of colors, with body colors from white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. The play of color can include any combination of these.
Opals range from transparent to opaque, with the transparency level often affecting the value and the type of opal.
Since opal is isotropic, it does not exhibit pleochroism. All colors in opal are visible from all angles, contributing to its unique visual properties.
Some opals may show fluorescence under ultraviolet light, with the presence of fluorescence typically seen in white or black opals, showing a blue to greenish glow.
Opal is generally considered to have fair to poor toughness due to its higher water content and lack of crystal structure, making it prone to cracking or “”crazing.””
The tenacity of precious opal is generally brittle, which makes it sensitive to stress and impact.
Being isotropic, precious opal has the optic sign of a singular index.
Precious opal does not typically show a characteristic absorption spectrum due to its amorphous structure, but when present, it can help in identifying the presence of certain elements.
Precious 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%.
While not common, some opals may exhibit chatoyancy, or the cat’s eye effect, when cut into a cabochon and viewed under direct light.
Asterism, or the star effect, is rare in opals but can occur when inclusions like needles intersect at various angles.
One of opal’s most desired features is its iridescence, where the stone shows many colors that change and move with the angle of observation and light.
Opal is typically non-magnetic, which aligns with its composition predominantly being hydrated silica.
Being a form of silica, precious opal is an insulator and does not conduct electricity effectively.
Precious opal generally exhibits no radioactivity, making it safe for frequent wear and use in various applications.
