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Pinpoint opal is a rare variety of precious opal characterized by minute points of spectral color flash against a dark or light background matrix. The phenomenon occurs when uniform, microscopic spheres of silica create a three-dimensional diffraction grating that produces distinct pinpoints of spectral color play.
These opals form in volcanic and sedimentary environments where silica-rich solutions penetrate void spaces. The pinpoint pattern emerges from the precise arrangement of silica spheres measuring between 150-400 nanometers in diameter, stacked in a cubic or hexagonal close-packed structure. The distinctive pinpoint pattern displays individual color points that maintain their position when the stone is rotated, unlike broader play-of-color patterns found in other opal varieties.
The most valuable specimens exhibit bright, distinct points of red, blue, and green against a black potch background, with points maintaining consistent intensity across the stone’s surface. These gems primarily originate from Australian deposits, particularly Lightning Ridge in New South Wales, where geological conditions created optimal circumstances for pinpoint pattern formation.
Pinpoint opal is typically a natural gemstone.
Common names for Pinpoint opal include Harlequin opal, Pinfire opal, and Flecked opal.
Pinpoint Opal, like other types of opals, typically has a hardness of 5.5 to 6 on the Mohs scale. This indicates that while it’s relatively softer compared to many other gemstones, it requires careful handling to avoid scratches and abrasions.
The refractive index of pinpoint opal ranges from about 1.44 to 1.46, which is relatively low compared to many other gemstones. This property affects how much the light bends when entering the gem.
Pinpoint Opal exhibits a vitreous to pearly luster. The presence of pinpoint inclusions can influence the overall luster, giving it a unique appearance.
Opals, including pinpoint opal, generally do not have any cleavage. This means they do not break along well-defined planes.
The fracture in pinpoint opal is typically conchoidal (shell-like), which is a common characteristic of more brittle gemstones.
The specific gravity of pinpoint opal ranges approximately from 1.98 to 2.25. This measurement can vary slightly depending on the structure and water content of the stone.
Pinpoint opal is isotropic, meaning it has no double refraction. Light travels through isotropic materials at the same speed regardless of the light’s direction.
Opals are famous for their play of color or “”fire,”” and this can include pinpoint opal depending on the pattern and size of color flashes. The dispersion is generally not high, but the color play can be quite striking.
Pinpoint opal is amorphous, meaning it does not have a crystalline structure. This lack of a regular atomic structure is what allows for the unique play-of-color for which opals are known.
The body color of pinpoint opal can vary widely, including white, black, blue, green, and pink. The small inclusions or pinpoint effects can add to the depth and richness of the color display.
The transparency of pinpoint opal can range from opaque to translucent. The level of transparency affects the play of color and the overall visual appeal of the gemstone.
Opal, including pinpoint opal, does not exhibit pleochroism due to its isotropic nature.
Some opals may show fluorescence under UV light, generally emitting a green or white glow; however, this can vary.
Opal is generally categorized as a fragile gemstone due to its high water content and brittleness, making it prone to cracking and chipping.
Pinpoint opal is brittle, making it susceptible to damage if exposed to harsh conditions, such as sharp blows or extreme temperature changes.
Being isotropic, pinpoint opal has an optic sign of uniaxial.
Typically, pinpoint opal does not show a distinctive absorption spectrum due to its amorphous nature and isotropic properties.
Opal is a hydrated form of silica (SiO₂·nH₂O). The water content can range from 3% to 21% by weight, but typically around 6% to 10%.
While not commonly observed in pinpoint opal, chatoyancy can occur if the stone has aligned fibrous inclusions.
Asterism is not typically associated with pinpoint opal as it requires a specific internal structure, such as aligned needle-like inclusions, which are not present in opals.
The play of color or iridescence in pinpoint opal is one of its most desired features, resulting from the diffraction of light by sub-microscopic spheres of silica.
Pinpoint opal is not magnetic.
Being a type of silica, pinpoint opal is an insulator and does not conduct electricity.
Opals, including pinpoint opal, are not radioactive.
