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Potch opal is a common variety of non-precious opal lacking the characteristic play-of-color found in precious opal, consisting primarily of amorphous hydrated silica spheres arranged in a non-diffractive pattern. The mineraloid occurs in various colors from white to gray, brown, or black, displaying no spectral phenomena due to its internal structure’s random arrangement.
The formation of potch opal results from silica-rich solutions penetrating host rocks under specific geological conditions, typically in sedimentary or volcanic environments. Its chemical composition (SiO2·nH2O) contains varying amounts of water, ranging from 6% to 10%. The material often forms as a matrix surrounding precious opal or appears in distinct layers within opal deposits.
Major deposits of potch opal exist in Australia’s Great Artesian Basin, particularly in Lightning Ridge, Coober Pedy, and Andamooka. The material serves as an indicator of precious opal deposits and finds use in lapidary arts as an inexpensive alternative to precious opal, though its market value remains significantly lower due to its lack of iridescence.
Potch opal is typically a natural gemstone.
Common names for Potch opal include common opal, potch, and jelly opal.
Potch opal, a common variety of opal that lacks the color play seen in precious opals, typically has a hardness of about 5.5 to 6.5 on the Mohs scale. This makes it relatively softer compared to many other gemstones, and thus more susceptible to scratches and abrasion.
The refractive index of potch opal ranges from approximately 1.44 to 1.46, which is relatively low compared to other gemstones. This contributes to its subtle luster.
Potch opal exhibits a waxy to dull luster, lacking the vibrant shine and play of color that characterizes precious opals.
Potch opal does not exhibit any cleavage, which is a common trait in the opal family.
This type of opal usually has a conchoidal fracture, giving it a smooth curved surface when broken.
The specific gravity of potch opal typically ranges from about 2.15 to 2.20, which is relatively light compared to many gemstones.
Opal, including potch opal, is isotropic and does not exhibit birefringence.
Potch opal generally does not show dispersion or “”fire”” due to its lack of color play, differentiating it from precious opal.
Opal, including potch opal, is amorphous, meaning it does not have a crystalline structure.
Potch opal can appear in various colors including white, gray, black, and occasionally a muted brown. However, it is primarily characterized by its lack of the vibrant color play seen in precious opals.
Potch opal can range from opaque to translucent, but it typically does not reach the transparency of some precious opals.
Being isotropic, potch opal does not exhibit pleochroism.
Fluorescence in potch opal can vary, but generally, it can show a weak to moderate green, white, or blue glow under ultraviolet light, depending on the impurities present.
Potch opal is generally considered to have fair to good toughness, making it reasonably resistant to breaking and chipping.
The tenacity of potch opal is considered brittle.
As an isotropic material, potch opal has an optic sign of “”isotropic””.
Potch opal does not typically show a distinctive absorption spectrum, unlike some other gemstones.
Potch opal is composed of hydrated silicon dioxide (SiO2·nH2O), similar to all types of opal.
This visual effect is generally absent in potch opal, as it lacks the necessary fibrous inclusions.
Like chatoyancy, asterism is not typically observed in potch opal.
Unlike precious opal, potch opal does not typically display iridescence.
Potch opal is non-magnetic.
Being primarily composed of silicon dioxide, potch opal is an electrical insulator.
Potch opal is not radioactive.
Potch opal, while lacking the dramatic visual effects of precious opal, still offers an intriguing option for those who appreciate the more subtle qualities of gemstones.
