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Opalized fossil is a unique form of fossilization where organic material has been replaced by opal, a hydrated amorphous silica mineral. The process occurs when silica-rich water permeates organic remains, gradually replacing the original tissue with opal while maintaining the specimen’s detailed morphological structure.
Australian opalized fossils represent the world’s finest examples, formed during the Cretaceous Period when inland seas covered vast regions of the continent. These specimens include marine reptiles, dinosaurs, mollusks, and plant matter, preserved in precious opal displaying vibrant play-of-color. The Coober Pedy and Lightning Ridge deposits yield rare specimens where the original bone structure exhibits spectacular iridescence through crystalline diffraction.
The mineralogical transformation preserves microscopic details of the original organism’s cellular structure, enabling paleontologists to study ancient life forms while providing valuable gemological specimens. These fossils command premium prices in both scientific and commercial markets, with exceptional specimens displaying vivid spectral colors across the visible spectrum.
Opalized fossil is typically a natural gemstone.
Common names for Opalized fossil opal include opalized wood, opalized ammonite, opalized belemnite, opalized coral, and opalized dinosaur bone.
Opal, which constitutes opalized fossils, typically has a hardness rating of about 5.5 to 6.5 on the Mohs scale. This makes it relatively softer compared to many other gemstones, which requires careful handling to avoid scratching.
Opalized fossils, made primarily of opal, have a refractive index ranging from approximately 1.37 to 1.52. This variance in RI contributes to the unique play of color seen in opal.
Opalized fossils exhibit an opalescent luster, which is a significant characteristic of opal. This luster gives them a beautiful, glowing appearance that seems to come from within the stone.
Opal has no cleavage, which is typical for this type of mineraloid. This means that opal does not break along defined planes.
Opal typically displays a conchoidal to uneven fracture. This type of fracture can create a smooth curved surface when the stone is broken.
The specific gravity of opal ranges from about 1.98 to 2.25. This can vary slightly depending on the type of opal and the inclusion within the fossil.
Opal is isotropic and shows no double refraction. Being amorphous, opal does not have a crystalline structure that would support birefringence.
Opal is particularly famed for its dispersion or “”fire,”” which is the play of color caused by the diffraction of light. This characteristic can vary depending on the formation and conditions of the specific opalized fossil.
Being amorphous, opal does not belong to any crystal system. This means that it does not have a regular atomic structure that defines crystalline materials.
Opalized fossils can range in color depending on the type of opal and the minerals that may be included in the fossil. Colors can include white, black, blue, green, red, and a myriad of others, often displaying multiple colors simultaneously.
Opals can range from completely opaque to translucent. The transparency of an opalized fossil depends on the type of opal and its structure.
Opal does not exhibit pleochroism due to its isotropic nature. Pleochroism is the ability to show different colors when viewed from different directions, typically observed in anisotropic materials.
Some opals may show fluorescence under ultraviolet light, typically displaying a green or blue glow. However, this property can vary widely among individual stones.
Opal is generally considered to have fair to poor toughness due to its brittleness and high water content, making it prone to cracking or breaking.
The tenacity of opal is generally categorized as brittle, which is typical for stones with higher water content and amorphous structures.
Being isotropic, opal does not have an optic sign.
Opals do not typically show a characteristic absorption spectrum due to their amorphous nature.
Opal is primarily composed of silica (SiO2) and water, with a chemical composition of hydrated silicon dioxide, often containing between 3% and 21% water by weight.
Some opals can exhibit chatoyancy, or the cat’s eye effect, when cut properly and when fibrous inclusions are present. This effect is seen as a single bright band of light across the surface of the gemstone.
Asterism, or the star effect, is less common in opals but can occur if the stone has inclusions of minerals that reflect light in a star-like pattern.
Opal is well-known for its iridescence, showcasing a spectacular play of color. This optical phenomenon is due to the diffraction of light by the silica spheres within its structure.
Opal is generally non-magnetic.
Opal has low electrical conductivity due to its composition primarily of silica and water.
Opalized fossils are typically not radioactive and are safe to handle and wear as jewelry.
