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Onyx opal is a rare variety of common opal exhibiting a black base color with play-of-color flashes, formed through the natural deposition of silica spheres within volcanic rock matrices. The gemstone displays distinctive optical phenomena characterized by iridescent flashes of spectral colors against its dark background.
The formation process occurs when hydrated silica gel infiltrates volcanic rock cavities under specific geological conditions, typically at temperatures below 100°C. The structural arrangement of uniform silica spheres creates a three-dimensional diffraction grating, producing the characteristic opalescence. The black background color derives from trace amounts of carbon, iron oxide, or manganese oxide inclusions.
Onyx opal measures 5.5-6.5 on the Mohs hardness scale and possesses a refractive index between 1.44-1.46. The gem’s specific gravity ranges from 1.98-2.20, with variable water content between 6-10%. Primary deposits are found in Lightning Ridge, Australia, with minor occurrences in Brazil and Indonesia.
Onyx opal is typically a natural gemstone.
Common names for Onyx opal include Black Opal, Banded Onyx, and Onyx Marble.
Onyx Opal, being a variety of opal, typically has a hardness rating of 5.5 to 6.5 on the Mohs scale. This makes it relatively softer compared to many other gemstones, which means it requires careful handling to avoid scratching.
The refractive index of Onyx Opal typically ranges from 1.37 to 1.52, which contributes to its unique play of color and brilliance.
The luster of Onyx Opal can range from waxy to vitreous. This characteristic helps in giving the gemstone an appealing glow that enhances its visual attractiveness.
Onyx Opal does not display any cleavage, which is a typical characteristic of opals in general. This means the gem does not have natural planes along which it splits easily.
Opals, including Onyx Opal, typically have a conchoidal fracture, creating smooth, curved surfaces when the stone is broken.
The specific gravity of Onyx Opal generally ranges from about 2.15 to 2.20, which is considered standard for opals.
Onyx Opal, like other forms of opal, is amorphous and does not exhibit birefringence or double refraction due to its lack of a crystalline structure.
Opals are well-known for their colorful play of light, known as “”fire,”” and Onyx Opal is no exception. The dispersion varies depending on the formation and conditions of each specific stone.
Being an amorphous mineraloid, Onyx Opal does not belong to any crystal system.
Onyx Opal typically displays a solid black or very dark background, which enhances the visibility of its colorful play-of-color.
The transparency of Onyx Opal can vary from opaque to translucent.
Onyx Opal typically does not exhibit pleochroism due to its amorphous nature and isomorphism.
Opals can sometimes show fluorescence under ultraviolet light, with varying intensity and color depending on the individual characteristics of the stone.
Onyx Opal has fair to good toughness, but it can be prone to cracking or chipping due to its relatively high water content and overall structure.
Opals, including Onyx Opal, are generally brittle and need to be handled with care to avoid damage.
As an amorphous substance, Onyx Opal does not have an optic sign.
Onyx Opal does not typically display a characteristic absorption spectrum due to its amorphous nature.
Onyx Opal is primarily composed of silicon dioxide with a significant percentage of water (up to 20%).
Rare instances of Onyx Opal may exhibit chatoyancy, but this is uncommon and depends on the internal structure and alignment of inclusions.
Asterism is not typically observed in Onyx Opal.
The iridescence in Onyx Opal is one of its most appealing features, often showing vibrant flashes of color that change with the angle of light.
Onyx Opal is not magnetic.
Opals, including Onyx Opal, generally exhibit poor electrical conductivity due to their amorphous structure and high water content.
Onyx Opal is not radioactive.
