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Prase opal is a rare variety of common opal displaying a distinctive leek-green to apple-green coloration, caused by microscopic inclusions of nickel-rich chlorite minerals within its amorphous silica structure. This cryptocrystalline form of hydrated silicon dioxide exhibits a characteristic internal play of color distinct from precious opal, presenting a uniform translucent to opaque appearance with a vitreous to waxy luster.
The mineral forms in nickel-rich serpentinite host rocks through low-temperature hydrothermal processes, predominantly found in specific localities including Tanzania, Poland, and California. Its hardness ranges from 5.5 to 6.5 on the Mohs scale, with a specific gravity between 2.0 and 2.2. The stone’s unique chromium and nickel content creates its signature green hue, differentiating it from other opal varieties through both chemical composition and optical properties.
Prase opal is typically a natural gemstone.
Common names for Prase opal include green opal, praseolite, and chrysopal.
Prase opal, also known as moss opal due to its green coloration, has a hardness of approximately 5.5 to 6.5 on the Mohs scale. This makes it relatively soft compared to many other gemstones, and somewhat susceptible to scratches and abrasion.
The refractive index of prase opal typically ranges from 1.44 to 1.46, which is relatively low, contributing to its subtle luster rather than a brilliant sparkle.
The luster of prase opal is commonly waxy to vitreous, giving it a soft and appealing glow that is less shiny compared to more refractive gems.
Prase opal does not exhibit any natural cleavage, making it somewhat more resilient in terms of shaping and polishing compared to gems with perfect cleavage.
This gemstone typically has a conchoidal to uneven fracture, which is characteristic of opals, reflecting its amorphous and brittle structure.
The specific gravity of prase opal is about 2.15, which is relatively low, indicating that the gem is not particularly dense.
Being an amorphous material, prase opal does not exhibit double refraction. It has an isotropic nature due to its lack of a crystalline structure.
Prase opal shows very low dispersion, resulting in minimal fire. This means the gemstone does not display significant rainbow colors when exposed to light, unlike more dispersive gems.
Prase opal does not have a crystal system as it is amorphous. This means it lacks a regular atomic structure typical of crystalline materials.
The color of prase opal is typically green, often with inclusions or veining that resembles moss, hence its nickname “”moss opal.””
Prase opal can range from opaque to translucent. The level of transparency often affects the visual appeal and value of the stone.
Pleochroism is not observed in prase opal due to its isotropic, amorphous nature.
Prase opal may exhibit weak to moderate fluorescence under UV light, typically showing green or white colors.
The toughness of prase opal is generally fair, but it can be prone to cracking or chipping due to its relative softness and brittleness.
The tenacity of prase opal is brittle, which is common for opals, meaning it can break or crack under stress.
Being isotropic, prase opal does not have an optic sign, as it does not doubly refract light.
The absorption spectrum for prase opal generally does not show distinctive features due to its amorphous nature and lack of strong color-causing elements.
Prase opal is primarily composed of silicon dioxide (SiO2) with water content, which can vary significantly, typically between 3% and 21%.
Chatoyancy is rarely observed in prase opal due to its typical opacity and lack of aligned fibrous inclusions necessary for this effect.
Asterism, or the star effect, is not typically seen in prase opal, as this effect also requires aligned crystalline inclusions, which are absent in this amorphous gem.
Some specimens of prase opal might display iridescence, particularly if they contain layers or inclusions that diffract light.
Prase opal is not magnetic and does not respond to magnetic fields.
As a type of opal, prase opal is generally a poor conductor of electricity, consistent with its composition as a non-metallic mineral.
Prase opal is not radioactive and poses no radiological hazards.
