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Semi black opal is a rare variety of opal exhibiting play-of-color against a dark gray to semi-black body tone, composed of hydrated silica dioxide with 6-10% water content, formed in sedimentary rocks under specific geological conditions. The gemstone displays a distinctive color play caused by the diffraction of light through regularly arranged spheres of silica, creating vivid spectral colors against its darker background.
The semi black variety originates primarily from Lightning Ridge, New South Wales, Australia, where it forms in ancient seabeds under unique geological conditions involving silica-rich solutions and specific pressure-temperature parameters. Its body tone ranges between N4 and N6 on the brightness scale, lighter than black opal (N1-N4) but darker than crystal opal. The stone’s internal structure consists of uniformly sized silica spheres arranged in a three-dimensional grid pattern, producing iridescent optical effects through Bragg diffraction.
Semi black opal is typically a natural gemstone.
Common names for Semi Black Opal include Black Crystal Opal, Dark Opal, and Semi Black Crystal Opal.
Semi black opals typically have a hardness of 5.5 to 6.5 on the Mohs scale. This makes it relatively softer than many other gemstones, and it is susceptible to scratching if not handled or stored properly.
The refractive index of semi black opals ranges from about 1.37 to 1.52. This variance is due to the differing proportions of silica and water in each specific opal.
Semi black opals exhibit a subvitreous to waxy luster, which contributes to their unique and mesmerizing visual appearance.
Opals, including semi black opals, generally do not have cleavage. This means they do not break along defined planes and typically fracture instead.
Semi black opals usually exhibit a conchoidal to uneven fracture. This is characteristic of materials that are relatively brittle and lack cleavage.
The specific gravity of semi black opals is approximately 1.98 to 2.25. This parameter helps in identifying genuine opals and distinguishing them from imitations.
Opals, including semi black opals, are amorphous and do not exhibit double refraction. They are isotropic gems.
Semi black opals are known for their splendid play of color or “”fire,”” which can show a wide spectrum of colors. The dispersion contributes significantly to their beauty and value.
Being amorphous, semi black opals do not have a crystal system. They are made up of silica spheres arranged in a random, grid-like pattern.
Semi black opals display a dark body tone which can range from dark gray to black, providing a stunning contrast to the vibrant play of color typical of opals.
These opals range from opaque to translucent, with the level of transparency affecting the play of color visibility and overall aesthetic appeal.
Pleochroism is not observed in semi black opals due to their amorphous nature.
Semi black opals may exhibit weak to moderate fluorescence under UV light, usually showing green or blue colors.
Opals are generally classified as fragile gems. Semi black opals, like other opals, have fair to good toughness but are vulnerable to drying, cracking, and being scratched.
The tenacity of semi black opals is generally brittle, which is a common characteristic of opals due to their internal structure and high water content.
Being isotropic, semi black opals have an optic sign of “”none”” as they do not doubly refract light.
Semi black opals do not typically show a characteristic absorption spectrum due to their amorphous nature and varied composition.
Semi black opals are primarily composed of silicon dioxide (SiO2) and water, with the water content varying between 3% and 21%.
Chatoyancy is uncommon in semi black opals, although in very rare cases, some stones may display a weak cat’s eye effect.
Asterism, or the star effect, is not typically seen in semi black opals.
The iridescence or play of color is a defining characteristic of semi black opals, presenting an array of colors that change and shift based on the viewing angle.
Semi black opals are not magnetic.
These opals are generally non-conductive due to their composition primarily of silica and water.
Semi black opals are not radioactive and pose no health hazards in this regard.
