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Bandfire opal is a premium variety of precious opal distinguished by sequential bands of spectral color that create a rolling fire effect across its surface. The gem forms in the sedimentary deposits of Lightning Ridge, New South Wales, through a process of silica-rich solution solidification under specific geological conditions.
The stone’s optical phenomenon results from an ordered array of uniformly-sized silica spheres, measuring 150-300 nanometers, which create a three-dimensional diffraction grating. These spheres organize into parallel layers that produce distinct bands of spectral colors, predominantly displaying red, blue, and green wavelengths.
The gem exhibits a dark body tone, N1-N4 on the opal brightness scale, with directional play-of-color that moves in consistent patterns when the stone is rotated. This distinctive optical property, combined with its limited availability in sizes above 5 carats, places Bandfire opal among the most sought-after varieties in the precious opal category.
Bandfire opal is typically a natural gemstone. It is not commonly synthesized in laboratories due to the unique and intricate play of color combined with its banding, which are difficult to replicate artificially.
Bandfire opal is also known as banded fire opal or layered fire opal.
The hardness of Bandfire opal on the Mohs scale ranges from 5.5 to 6.5, making it relatively softer compared to many other gemstones. This softness requires careful handling and setting in jewelry.
Bandfire opal has a refractive index typically between 1.37 and 1.52. This property contributes to its striking play of color.
The luster of Bandfire opal is usually waxy to subvitreous, which adds to its visual appeal by giving a soft sheen rather than a sharp reflection.
Bandfire opal does not exhibit any natural planes of cleavage, which is a common characteristic of opals in general.
This type of opal often displays a conchoidal fracture, which is smooth and curved, similar to the breakage pattern of glass.
The specific gravity of Bandfire opal can vary from about 1.98 to 2.25, which is typical for opals, indicating a relatively low density compared to other crystalline gemstones.
Bandfire opal is an amorphous mineral and does not show birefringence. It is isotropic, meaning it has the same optical properties in all directions.
One of the hallmark characteristics of Bandfire opal is its dispersion or “fire,” which refers to the splitting of light into multiple spectral colors, creating a dazzling visual effect.
Being an amorphous solid, Bandfire opal does not have a crystal system. Its internal structure lacks the regular, repeating pattern found in crystalline minerals.
The colors in Bandfire opal are vibrant and can include a wide range from reds and oranges to greens and blues, usually displayed in a play of color pattern that is enhanced by its banding.
Bandfire opals can range from being opaque to translucent, depending on the conditions under which they were formed and the specific composition of the stone.
Pleochroism is not observed in Bandfire opal due to its isotropic nature.
Some Bandfire opals may exhibit fluorescence under ultraviolet light, typically showing colors like green or white depending on the individual stone’s properties.
The toughness of Bandfire opal is generally considered to be fair to good, although opals are prone to drying out and may crack or craze if subjected to sudden changes in temperature or humidity.
Bandfire opal is brittle, which is a common trait among opals due to their internal structure and higher water content.
As an isotropic gemstone, Bandfire opal shows no optic sign.
Bandfire opal typically does not display a distinctive absorption spectrum.
The chemical formula for Bandfire opal is SiO2·nH2O, indicating that it is a hydrated form of silica.
Chatoyancy or the cat’s eye effect is generally not observed in Bandfire opal, due to its lack of fibrous structures and the nature of its internal play of color.
Bandfire opal does not exhibit asterism, which is generally seen in gemstones with a more regular crystalline structure and specific inclusions.
The iridescence in Bandfire opal is one of its most striking features, often shown as a brilliant play of colors that move and shift under different angles of light.
Bandfire opal is not magnetic.
As an amorphous form of silica, Bandfire opal has low electrical conductivity.
Bandfire opal is not radioactive and poses no health hazards from radiation.
