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Opalized shell is a rare fossilized formation where ancient mollusk shells have been replaced by precious opal through a mineralization process spanning millions of years. The original calcium carbonate structure of the shell transforms into hydrated silica gel, preserving the shell’s intricate patterns while displaying the characteristic play-of-color found in precious opal.
These specimens form in marine sediments when silica-rich solutions infiltrate the void left by decomposed shell material. The most significant deposits occur in Australian opal fields, particularly in Coober Pedy and Andamooka, where Cretaceous period marine fossils create spectacular gemological specimens. The shells exhibit both external and internal opalescence, often preserving microscopic details of growth lines and surface textures from the original mollusk.
The scientific value of opalized shells extends beyond gemology into paleontology, providing crucial data about ancient marine ecosystems and the chemical processes of fossilization. These specimens frequently display brilliant spectral colors across the visible spectrum due to their unique diffraction grating structure.
Opalized shell is typically a natural gemstone.
Common names for Opalized shell opal include shell opal, opalized fossil shell, and boulder opal shells.
Opalized shell, being a type of opal, typically has a hardness of 5.5 to 6.5 on the Mohs scale. This indicates a relatively softer gem compared to many others, which makes it somewhat susceptible to scratches.
The refractive index for opalized shells is generally in the range of 1.37 to 1.52. This variation depends on the specific opal type and its formation.
Opalized shells exhibit a distinctive luster that can range from waxy to subvitreous. When polished, they can display a beautiful, gleaming surface that enhances their visual appeal.
Opal, including opalized shells, typically does not exhibit any cleavage. This is a characteristic that makes them different from many crystalline gemstones.
Opalized shells usually have a conchoidal to uneven fracture, which is typical of opaline silica material. These types of fractures can create appealing natural patterns, enhancing the stone’s visual interest.
The specific gravity of opalized shells usually ranges from about 1.98 to 2.25, which is consistent with other types of opals.
Opalized shells are amorphous and do not exhibit double refraction. This is a characteristic feature of the amorphous structure of opal.
Opalized shells can display a remarkable play of color or “”fire,”” which is caused by the diffraction of light passing through the microscopic silica spheres within the structure of the opal.
Being an amorphous mineraloid, opalized shells do not belong to any crystal system. They do not have a crystalline structure which is typical in other gemstones.
The color of opalized shells can vary widely, including white, black, blue, green, red, and a mix of these colors, displaying vibrant and iridescent hues.
Opalized shells can range from opaque to translucent. The level of transparency often affects the play of color and the overall appearance of the gemstone.
As an isotropic substance, opalized shells do not exhibit pleochroism. The color play visible in opal is due to light interference from its internal structure rather than color variation with angle.
Opals, including opalized shells, can sometimes exhibit fluorescence under ultraviolet light, typically showing green or white colors.
Opalized shells have fair to good toughness, but they can be prone to cracking or chipping due to their relatively lower hardness and potential internal stresses.
The tenacity of opalized shells can be described as brittle, which is a common characteristic of opals due to their amorphous structure and water content.
Opalized shells are isotropic and do not have an optic sign due to their amorphous nature.
Opalized shells typically do not show a distinctive absorption spectrum, unlike many crystalline gemstones.
Opalized shells primarily consist of silicon dioxide (SiO2) with a variable amount of water content. This composition can include other elements depending on the specific environmental conditions during formation.
While less common than in some other gemstones, some opalized shells can exhibit chatoyancy, especially when cut into cabochons and aligned properly.
Asterism or the star effect is generally not observed in opalized shells due to their amorphous structure and typical cutting styles.
One of the most appealing features of opalized shells is their iridescence. This optical phenomenon creates vibrant, shifting colors that change with the angle of light reflection.
Opalized shells are generally non-magnetic, consistent with their silica composition.
Being mostly composed of silica, opalized shells are poor conductors of electricity.
Opalized shells are not radioactive, making them safe to handle and wear in jewelry.
