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Freeform opal is a naturally occurring hydrated amorphous form of silica, shaped through non-traditional cutting techniques that follow the stone’s natural contours rather than conventional faceting patterns. This distinctive gemstone exhibits play-of-color through a phenomenon called diffraction, where light interacts with microscopic silica spheres arranged in a regular pattern.
The cutting style preserves the opal’s maximum carat weight and optimal color display by working with the stone’s natural formation patterns and color zones. These specimens often retain their organic shape, featuring smooth, undulating surfaces that showcase the gem’s internal structure and color play. Freeform opals frequently display potch (common opal) backing, which provides stability and enhances the stone’s durability.
Australian freeform opals, particularly those from Lightning Ridge and Coober Pedy, are renowned for their exceptional quality and distinctive characteristics, including rare black body tones and vivid spectral colors. These gems contain 6-10% water content within their chemical structure, contributing to their unique optical properties and requiring specific care parameters for long-term preservation.
Freeform Opal is typically a natural gemstone.
Common names for Freeform Opal include Boulder Opal, Matrix Opal, Andamooka Opal, and Crystal Opal, among others that describe its unique shapes and patterns rather than conforming to standard opal classifications.
Freeform opal has a hardness of 5.5 to 6.5 on the Mohs scale, making it relatively soft compared to many other gemstones. This means it can be scratched by harder substances and should be worn and stored carefully to avoid damage.
The refractive index of opal generally ranges from 1.37 to 1.52. This variable index is due to opal’s unique composition of silica spheres and water.
Opals exhibit a vitreous to waxy luster, which contributes to their beautiful and appealing visual properties, especially when polished.
Opal does not have a cleavage plane, which means it does not break along a defined plane of weakness. This characteristic can make it challenging to cut and shape.
Opal fractures in a conchoidal manner, similar to glass, often resulting in a smooth surface.
The specific gravity of opal ranges from about 1.98 to 2.25. This variability is largely influenced by its water content, which can fluctuate over time and with environmental changes.
Opal is an isotropic material and does not exhibit double refraction. It has a single refractive index due to its amorphous character.
Opal is particularly famous for its dispersion or “”fire,”” which appears as flashes of colored light. This effect is caused by the diffraction of light passing through the microscopic silica spheres within its structure.
Opal is amorphous, meaning it does not have a crystalline structure. This is a key characteristic that differentiates it from crystalline gemstones.
Freeform opal can display a wide range of colors, including white, black, blue, green, red, and orange. The color can vary significantly due to the gemstone’s play-of-color phenomenon.
Opals can range from completely opaque to translucent. The level of transparency affects the visual appearance and value of the opal.
Opal does not exhibit pleochroism due to its amorphous structure, which means its color remains constant regardless of the viewing angle.
Some opals may exhibit fluorescence under ultraviolet light, typically showing a white or greenish glow, depending on the type of opal and its trace elements.
Opal is generally considered to have fair to poor toughness due to its high water content and lack of a crystalline structure, making it susceptible to cracking and chipping.
Opal’s tenacity is generally categorized as brittle, which is typical for an amorphous mineral.
Being isotropic, opal has no optic sign as it does not double refract.
Opal generally does not produce a characteristic absorption spectrum due to its amorphous nature and variable composition.
Opal is composed of silicon dioxide (SiO2) and water, with its chemical formula often represented as SiO2·nH2O.
Some opals can exhibit a cat’s eye effect if they possess aligned fibrous inclusions. However, this is relatively rare compared to other gemstones like chrysoberyl.
Star opals are even rarer than those showing chatoyancy and occur when the stone displays a star-like pattern, typically resulting from included crystals or fibers.
Iridescence is one of the defining characteristics of opal, particularly noted in precious opals where light is diffracted by the microscopic layers of silica spheres.
Opal is non-magnetic due to its lack of ferromagnetic minerals.
Opal exhibits poor electrical conductivity because of its amorphous nature and high water content.
Standard opal exhibits negligible to no radioactivity, making it safe for handling and wear.
