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Banded opal is a distinctive variety of common opal exhibiting parallel bands of varying colors arranged in horizontal layers, formed through sequential deposition of silica-rich solutions in fissures or cavities within host rocks. This mineraloid, composed of hydrated silica (SiO2·nH2O), displays no play of color like precious opal but presents striking alternating bands of different hues and opacities.
The formation process occurs when silica-rich solutions infiltrate rock cavities in successive waves, creating distinct layers as each solution solidifies. These bands range from translucent to opaque and commonly appear in combinations of white, blue, green, pink, and brown. The mineral’s hardness measures 5.5-6.5 on the Mohs scale, with a specific gravity of 1.9-2.2.
Notable deposits of banded opal exist in Mexico, Brazil, and the United States, particularly Oregon, where specimens often form in association with rhyolitic volcanic rocks. The material’s parallel banding pattern makes it highly sought after for cabochon cutting and ornamental objects.
Banded opal is typically a natural gemstone.
Common names for Banded Opal include Layered Opal, Ribbon Opal, and Striped Opal.
Banded Opal generally has a hardness rating of around 5.5 to 6.5 on the Mohs scale, making it relatively soft compared to many other gemstones. This means it can be susceptible to scratches and abrasion if not handled with care.
The refractive index of Banded Opal typically ranges from about 1.44 to 1.46, which contributes to its unique play of color and overall visual appeal.
Banded Opal exhibits a vitreous to pearly luster, giving it a glossy and sometimes iridescent appearance, particularly when polished.
Opal, including Banded Opal, does not have a defined cleavage. This means that it does not break along predictable planes, which can make cutting and shaping the gem more challenging.
Banded Opal is known for its conchoidal fracture, which results in a smooth, curved surface when the gemstone is broken.
The specific gravity of Banded Opal varies around 1.98 to 2.25, depending on the exact composition and the presence of impurities or inclusions.
Opal, such as Banded Opal, is isotropic and does not exhibit double refraction. This means that light rays passing through the gemstone are not split into two separate rays.
Banded Opal can display a notable level of dispersion or “”fire,”” which refers to the splitting of white light into the spectral colors. This is particularly visible in high-quality specimens.
Banded Opal crystallizes in the amorphous crystal system. It does not have a regular crystal structure, which is typical for crystalline materials.
The color of Banded Opal varies widely and can include layers or bands of white, black, blue, green, red, and yellow. The bands of color can be straight, curved, or irregular, adding to the stone’s visual interest.
Banded Opal ranges from opaque to translucent, which affects how it is used in jewelry and other decorative items.
Opal, including Banded Opal, does not exhibit pleochroism due to its isotropic nature.
Some varieties of Banded Opal may exhibit fluorescence under ultraviolet light, typically showing a white or green glow, although this can vary widely.
Opal is generally considered to have fair to good toughness, but Banded Opal can be somewhat fragile due to its natural inclusions and water content.
Banded Opal is brittle, which makes it susceptible to cracking or chipping if subjected to harsh impacts or pressure.
Being isotropic, Banded Opal has an optic sign of “”isotropic,”” meaning it has the same optical properties in all directions.
Banded Opal does not typically display a characteristic absorption spectrum, unlike some other gemstones.
Banded Opal is a form of silica (SiO2·nH2O) and contains varying amounts of water. The chemical composition can influence its physical properties and color.
While not common, some types of Banded Opal can exhibit chatoyancy, or the cat’s eye effect, when cut appropriately and if fibrous inclusions are present.
Asterism, or the star effect, is not typically observed in Banded Opal due to its amorphous structure and lack of aligned inclusions.
Banded Opal can display a beautiful range of iridescent colors, especially when viewed under light from different angles. This is one of its most appealing and distinctive features.
Banded Opal is not magnetic and does not respond to a magnetic field.
As a non-metallic mineral, Banded Opal has poor electrical conductivity.
Banded Opal is not radioactive and poses no health risks related to radioactivity.
