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Non precious opal is a hydrated amorphous form of silica (SiO2·nH2O) lacking the iridescent play-of-color characteristic found in precious opal. The mineral forms in sedimentary rocks through the precipitation of silica from low-temperature solutions, typically containing 6-10% water content by weight.
Common opal occurs in various colors including white, yellow, red, brown, and black, deriving its hues from trace elements and mineral inclusions. The material exhibits a vitreous to resinous luster with a hardness of 5.5-6.5 on the Mohs scale. Unlike its precious counterpart, non precious opal displays only a simple opalescence without spectral color diffraction, due to its irregular arrangement of silica spheres.
This variety of opal forms extensive deposits worldwide, with significant occurrences in Australia, Mexico, and the United States. The mineral commonly appears as nodules, veins, or cavity fillings in volcanic rocks, and can replace organic materials in petrified wood. Its formation requires specific geological conditions including moderate temperatures, appropriate pH levels, and sufficient silica concentration in groundwater.
Non precious opal is typically a natural gemstone.
Common names for Non precious opal include potch, common opal, and jelly opal.
Non-precious opal, generally referred to as common opal or potch, has a hardness rating of about 5.5 to 6.5 on the Mohs scale. This indicates that it is relatively softer than many other gemstones, making it more susceptible to scratching and wear.
The refractive index of non-precious opal ranges from approximately 1.44 to 1.46, which is relatively low compared to precious opals and other gemstones. This contributes to its distinct, subdued luster compared to the vibrant play of color seen in precious opals.
Common opal has a waxy to dull luster, lacking the characteristic play of color and iridescence observed in precious opals. The luster is more subdued due to its microcrystalline structure and absence of silica spheres that contribute to the opalescence in precious opals.
Non-precious opal does not exhibit any natural cleavage planes. It tends to break irregularly, contributing to its fragile nature during cutting, shaping, or drilling processes.
The typical fracture in non-precious opal is conchoidal or uneven, which is common in amorphous or cryptocrystalline mineral substances. This kind of fracture can make the gem difficult to work with in jewelry making.
The specific gravity of common opal varies approximately between 1.98 and 2.25, slightly lighter than precious opal. This property helps in distinguishing it from its more valuable counterparts.
Non-precious opal exhibits no double refraction due to its amorphous nature. It is an isotropic material, meaning that light passes through it with uniform velocity in all directions.
Common opal shows negligible dispersion or “”fire.”” This is a stark contrast to precious opal, which is highly valued for its colorful fire.
Opal is amorphous, meaning it does not have a crystalline structure. This absence of a crystal system differentiates it from crystalline gemstones.
Non-precious opal can come in a variety of colors including white, grey, blue, green, and pink. However, these colors tend to be more muted than those observed in precious opal.
Common opal ranges from opaque to translucent. Unlike precious opal, it rarely shows any level of transparency.
This gemstone exhibits no pleochroism due to its amorphous structure. Pleochroism is typically a feature of crystalline minerals.
Some non-precious opals may exhibit a weak fluorescence under UV light, typically showing green or yellow hues. However, this property is highly variable.
Opal has fair to good toughness, but its water content and structure can make it prone to cracking and chipping. It is more vulnerable to heat and pressure compared to more resilient gemstones.
Non-precious opal is generally brittle due to its amorphous and hydrated silica structure. This makes it less ideal for high-impact or high-wear jewelry settings.
Being isotropic, the optic sign of non-precious opal is not applicable as it does not doubly refract light.
Common opal does not typically display a characteristic absorption spectrum, which is more commonly associated with crystalline gemstones.
Non-precious opal is primarily composed of silica (SiO2) and water. It can contain between 6% and 10% water by weight.
Chatoyancy is rarely displayed in non-precious opal due to its lack of fibrous inclusions and aligned structures required to produce the cat’s eye effect.
Like chatoyancy, asterism is not observed in non-precious opal because it lacks the necessary aligned needle-like inclusions.
Non-precious opal does not typically exhibit iridescence. The iridescent effect seen in some opals is due to the diffraction of light by silica spheres, which are not present in common opal.
Common opal is not magnetic due to its chemical composition lacking magnetic elements.
Non-precious opal is generally an electrical insulator owing to its composition mainly of silica and water, both of which are poor conductors of electricity.
Non-precious opal is not radioactive. It is safe for handling and wearing without any health risks associated with radioactivity.
