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Fossil opal is a mineraloid formed when silica-rich solutions infiltrate and replace organic materials in fossils, creating a unique form of preservation where the original structure is maintained through opalescence. The process, known as opalization, occurs when hydrated silica gel fills microscopic voids in buried organic matter, resulting in a three-dimensional preservation that displays characteristic play-of-color.
These gemstones form primarily in sedimentary environments where rapid burial and specific geochemical conditions allow for the preservation of organic structures. Notable deposits exist in Australia’s Great Artesian Basin, where marine reptiles, dinosaurs, and invertebrates from the Cretaceous period have been transformed into precious opal. The internal structure consists of uniformly sized spheres of silica arranged in a regular pattern, creating diffraction gratings that produce spectacular spectral colors through Bragg diffraction.
Fossil Opal is typically a natural gemstone.
Common names for Fossil Opal include Opalized Wood, Opalized Fossil, Petrified Opal, and Opal Fossil.
Fossil opal, like other opal varieties, is relatively soft compared to many other gemstones, typically ranging between 5.5 and 6.5 on the Mohs scale. This makes it more susceptible to scratching and wear.
The refractive index of fossil opal can vary but generally falls between 1.44 and 1.46, which is characteristic of opals. This RI contributes to the gem’s appealing play of color.
Fossil opal exhibits a vitreous to pearly luster, depending on the conditions of formation and the presence of impurities or inclusions.
Opal, including fossil opal, does not have any cleavage planes. It is an amorphous mineraloid, meaning it does not have a crystalline structure that would facilitate cleavage.
Fossil opal typically exhibits a conchoidal fracture, which is a curved breakage surface reminiscent of broken glass. This type of fracture is common in amorphous and fine-grained minerals.
The specific gravity of fossil opal can range from approximately 1.98 to 2.25. This variation can depend on the type of opal and its water content.
As an amorphous solid, fossil opal does not exhibit double refraction. It is isotropic, meaning it has the same optical properties in all directions.
Opals are well-known for their dispersion or “”fire,”” which refers to the splitting of light into multiple spectral colors. Fossil opal can display this characteristic, though the intensity can vary.
Being amorphous, fossil opal does not belong to any crystal system.
Fossil opal can display a wide range of colors, often showing vibrant play of color against a white, gray, or black body color. The colors can include hues of green, blue, orange, and red.
The transparency of fossil opal can vary from opaque to translucent. Rare pieces might be transparent, but this is less common in fossil varieties.
Pleochroism is not observed in fossil opal due to its isotropic nature.
Some fossil opals may exhibit fluorescence under ultraviolet light, typically showing green or white colors. However, this property can vary significantly.
Fossil opal has fair to good toughness, but it can be prone to cracking due to its inherent water content and sensitivity to temperature changes.
Opal is generally brittle, and fossil opal is no exception. It can be sensitive to impact and pressure.
Being isotropic, the optic sign of fossil opal is uniaxial and non-directional.
Fossil opal might not have a distinctive absorption spectrum due to its amorphous nature. Any absorption features would primarily relate to the color and type of opal.
Fossil opal is primarily composed of silica (SiO2) and water. The water content can vary, typically between 3% and 21%.
Although rare, some fossil opals can exhibit chatoyancy, a ‘cat‚Äôs eye’ effect, when cut appropriately. This occurs when fibrous inclusions are aligned in parallel.
Asterism or the star effect is not commonly associated with fossil opal. It is more typical in other gemstones like star sapphire or star ruby.
Fossil opal can exhibit iridescence, especially when it has a significant play of color, which is a form of iridescence caused by the diffraction of light.
Fossil opal is generally non-magnetic.
As a non-metallic mineraloid, fossil opal has very low electrical conductivity.
Fossil opal is not radioactive.
