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Mountain opal is a rare variety of common opal found in volcanic mountainous regions, characterized by its translucent to opaque appearance and distinctive play of colors within a predominantly white or cream-colored matrix. This silica-based mineraloid forms in volcanic host rocks through the precipitation of silica-rich solutions in cavities and fissures at high altitudes.
The stone exhibits unique optical properties due to its microstructure of regularly arranged silica spheres, which create diffraction patterns resulting in iridescent displays. Mountain opals contain 6-10% water content, with a hardness of 5.5-6.5 on the Mohs scale. The gemstone’s formation occurs specifically in mountainous regions where hydrothermal activity has altered volcanic rocks, creating conditions for opal deposition at elevations typically above 1,000 meters. Notable deposits exist in the volcanic regions of Mexico, Honduras, and the Andes Mountains, where the combination of geological conditions and altitude produces specimens with distinctive banding patterns and color variations unique to high-altitude formations.
Mountain opal is typically a natural gemstone.
Common names for Mountain opal include: Hyalite, Water opal, and Muller’s glass.
Mountain opal, like other opal varieties, has a relatively low hardness rating on the Mohs scale, typically ranging between 5.5 and 6.5. This makes it somewhat susceptible to scratches and abrasion, which should be considered when used in jewelry that may be exposed to rough wear.
The refractive index of mountain opal generally ranges from 1.44 to 1.46, which is relatively low compared to many other gemstones. This contributes to its subtle play of color rather than a brilliant sparkle.
Mountain opal exhibits a vitreous to waxy luster, providing a soft and appealing glow that enhances its color play.
Opal, including mountain opal, typically does not exhibit any cleavage. This is a characteristic of the mineraloid, as it is amorphous and lacks a crystalline structure.
Mountain opal has a conchoidal fracture, which is typical for materials that are not crystalline. The edges of a broken piece will typically be smooth and curved.
The specific gravity of mountain opal ranges approximately from 1.98 to 2.25, varying slightly depending on the inclusion and water content.
Being an amorphous substance, mountain opal does not display birefringence. It is isotropic, meaning that light travels through the gem uniformly in all directions.
Opals are well-known for their colorful play-of-color, which is not to be confused with dispersion. Mountain opal can exhibit a beautiful play-of-color, which is caused by the diffraction of light passing through its silica sphere structure.
Mountain opal does not have a crystal system as it is amorphous. This lack of internal crystalline structure contributes to its unique visual properties.
The color of mountain opal can vary widely, including white, gray, blue, green, and pink. The presence of various trace elements and the conditions during formation contribute to its color diversity.
Mountain opals can range from opaque to translucent. The level of transparency affects the appearance of the play-of-color and the overall aesthetic value of the gemstone.
Mountain opal does not exhibit pleochroism due to its isotropic nature.
Some mountain opals may show fluorescence under UV light, typically displaying a greenish or whitish glow. This property varies depending on the specific composition of the opal.
Despite its relative softness, the toughness of mountain opal can be considered fair due to its amorphous structure, which can absorb shock better than many crystalline gemstones.
Mountain opal is generally brittle, which is a consideration for setting and wearing this type of gemstone in jewelry.
As an isotropic material, mountain opal has no optic sign.
Mountain opal does not typically show a characteristic absorption spectrum, as it is an amorphous solid.
Mountain opal is primarily composed of silica (SiO2) and water. It can contain up to 20% water, which may be lost over time, leading to cracking.
Some mountain opals may exhibit chatoyancy, or a cat’s eye effect, if they have aligned fibrous inclusions. However, this is less common than in other gemstones like chrysoberyl.
Asterism, or the star effect, is not typically seen in mountain opal due to its amorphous structure and the typical absence of the necessary aligned inclusions.
The play-of-color seen in many mountain opals is a form of iridescence arising from the microscopic structure of stacked silica spheres within the gem.
Mountain opal is not magnetic.
Being comprised mostly of silica, mountain opal has low electrical conductivity.
Mountain opal is not radioactive.
