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Belemnite opal is a rare fossilized form of opal that occurs when silica-rich solutions replace the calcite guards of extinct cephalopod mollusks known as belemnites during the mineralization process. The resulting gemstone maintains the distinctive bullet-shaped structure of the original belemnite fossil while displaying the characteristic play-of-color typical of precious opal.
These unique gems primarily originate from the opal fields of South Australia, particularly in Coober Pedy and Andamooka regions, where they formed during the Cretaceous period approximately 120 million years ago. The internal structure shows distinct growth rings and a radial pattern inherent to the original belemnite guard, creating a natural combination of paleontological specimen and precious gemstone. The opal replacement process preserves microscopic details of the original fossil structure while introducing the ordered arrangement of silica spheres that creates opal’s distinctive optical phenomena.
Belemnite opal is natural, originating from the fossilized remains of the ancient marine creature known as belemnite.
Common names for Belemnite opal include pipe opal, fossil opal, and opalised belemnite.
Belemnite opal, like other opals, typically has a hardness of around 5.5 to 6.5 on the Mohs scale. This moderate hardness means it is somewhat susceptible to scratches and abrasion.
The refractive index of belemnite opal generally ranges from approximately 1.44 to 1.46, which is characteristic of opals. This contributes to its unique play of color.
Belemnite opal exhibits a subvitreous to waxy luster, giving it a glow that enhances its visual appeal and depth.
Opal, including belemnite opal, typically does not have any cleavage, meaning it does not break along defined planes.
Belemnite opal often has a conchoidal fracture, which is a curved breakage surface resembling the shape of a shell.
The specific gravity of belemnite opal lies between approximately 2.15 and 2.21, which is fairly typical for opals.
Opal, including belemnite opal, is an isotropic gemstone, meaning it does not exhibit double refraction.
Opals are well-known for their dispersion or fire, which is the splitting of light into its spectral colors. Belemnite opal displays a vivid play of color due to its internal structure.
Belemnite opal, like all opals, is amorphous, meaning it lacks a crystalline structure and is composed of silica spheres.
The color of belemnite opal can vary widely, featuring a base color that can be white, yellow, red, blue, green, or black, with a play of color that can include multiple hues.
Belemnite opal can range from opaque to translucent. The level of transparency affects the visual effects and value of the gemstone.
Belemnite opal does not exhibit pleochroism due to its isotropic nature.
Some opals, including belemnite opal, may exhibit fluorescence under ultraviolet light, typically showing green or white colors.
Opal has fair to good toughness, but belemnite opal can be somewhat brittle and sensitive to changes in temperature and humidity.
Belemnite opal is generally brittle, which is a common characteristic of opals due to their internal structure and water content.
As an isotropic material, belemnite opal has an optic sign of “”isotropic,”” showing the same optical properties in all directions.
Belemnite opal does not typically display a characteristic absorption spectrum due to its amorphous nature.
Belemnite opal is primarily composed of silicon dioxide (SiO‚ÇÇ) with varying amounts of water (H‚ÇÇO).
Though rare in belemnite opal, some specimens can exhibit chatoyancy, which appears as a single bright, silky band of light across the surface.
Asterism, or the star effect, is generally not observed in belemnite opal.
Belemnite opal is particularly noted for its iridescence, displaying a variety of colors that seem to change and move under light.
Belemnite opal is generally non-magnetic due to its silicon dioxide composition.
Being composed primarily of silicon dioxide, belemnite opal is generally a poor conductor of electricity.
Belemnite opal is not radioactive and poses no radiological hazards.
Belemnite opal, with its distinct origin and mesmerizing color play, remains a cherished gemstone, particularly for collectors and enthusiasts of natural history.
