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Pinfire opal is a rare variety of precious opal exhibiting distinct pinpoint flashes of spectral colors against a transparent to translucent base. The phenomenon occurs when small, concentrated areas of color play create distinct points of fire throughout the gemstone’s structure.
The unique optical effect in pinfire opals results from the regular arrangement of uniformly sized silica spheres within the stone’s microstructure. These spheres, typically 150 to 300 nanometers in diameter, diffract light to produce vivid, isolated points of color that appear to float within the opal’s body. The most valued specimens display sharp, distinct points of red, which command premium prices in the gemstone market.
These gems predominantly originate from Australian deposits, particularly the Lightning Ridge region of New South Wales, where geological conditions created ideal circumstances for their formation. The stones form in the fissures of rocks when silica-rich water solutions slowly evaporate, leaving behind organized layers of silica spheres that create the characteristic pinfire effect.
Pinfire opal is typically a natural gemstone.
Common names for Pinfire opal include Fire Opal, Girasol Opal, and Harlequin Opal.
Pinfire opal has a hardness of about 5.5 to 6.5 on the Mohs scale, which indicates a moderate level of scratch resistance. This makes it relatively softer compared to many other gemstones and more susceptible to scratches and abrasion.
Pinfire opal typically has a refractive index in the range of 1.440 to 1.460, which is relatively low compared to many other gemstones. This property affects how much the light is bent or refracted when entering the stone.
The luster of pinfire opal is generally classified as subvitreous to waxy. This quality gives the opal a slightly glossy look that is less shiny compared to gemstones with a vitreous (glass-like) luster.
Opal, including pinfire opal, typically does not exhibit cleavage. This means that it does not have natural planes along which it splits easily.
Pinfire opal usually has a conchoidal fracture, which results in a smooth, curved surface when the gemstone is broken.
The specific gravity of pinfire opal lies typically between 2.15 and 2.21, which is considered low, indicating that opals are lighter compared to many other gemstones.
Pinfire opal is an amorphous gemstone and does not exhibit double refraction. It has an isotropic nature due to its amorphous structure.
Pinfire opal is well-known for its dispersion or fire, which appears as flashes of colored light. This optical phenomenon is particularly valued in opals and varies from stone to stone.
Being an amorphous mineraloid, pinfire opal does not have a crystal system. It lacks a defined crystalline structure, which differentiates it from crystalline forms of silica.
Pinfire opal displays a base color that can range from white, gray to black, and includes vibrant flashes of green, blue, red, and orange. The “”pinfire”” descriptor specifically refers to the small, pinpoint flashes or specks of bright color.
Pinfire opals can range from opaque to translucent. This variation affects the play of color and overall appearance of the gemstone.
Pleochroism is not observed in pinfire opal due to its isotropic nature as an amorphous solid.
Some opals, including pinfire opals, may exhibit fluorescence under ultraviolet light, typically showing a white or creamy glow. However, this property can vary greatly among individual stones.
Opal has fair to good toughness, but care should be taken as it can be brittle and sensitive to changes in humidity and temperature, which might cause it to crack or craze.
Pinfire opal is generally brittle, making it more susceptible to damage from impact or pressure compared to more tenacious gemstones.
As an isotropic material, pinfire opal does not have an optic sign associated with birefringent materials.
Pinfire opal does not typically display a characteristic absorption spectrum due to its amorphous nature and lack of regular internal structure necessary to create selective absorption of light.
Pinfire opal is primarily composed of silica (SiO2) and water. The water content can range from 3% to 21%, which significantly influences its properties.
While typically not known for chatoyancy, some opals might exhibit a cat’s eye effect if they have aligned fibrous inclusions.
Asterism or the star effect is generally not seen in pinfire opals. This effect is more common in other gemstones like star sapphires and star rubies.
Pinfire opal is particularly noted for its iridescence. The play of color occurs due to the diffraction of light passing through tiny silica spheres within the structure of the opal.
Pinfire opal is non-magnetic.
Being composed largely of silica and water, pinfire opal is a poor conductor of electricity.
Pinfire opal is not radioactive.
