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Precious fire opal is a hydrated amorphous form of silica (SiO2·nH2O) distinguished by its internal play of iridescent colors against a translucent to transparent body color, typically ranging from water-clear to milky white. The phenomenon, known as opalescence, occurs through the diffraction of light within a precise arrangement of silica microspheres.
Mexican fire opals, mined primarily in the state of Jalisco, exhibit the most intense red, orange, and yellow body colors due to traces of iron oxide within their chemical structure. These gems form in volcanic environments where silica-rich water penetrates rock cavities, eventually solidifying into opal under specific pressure and temperature conditions.
The Mohs hardness of fire opal ranges from 5.5 to 6.5, with a specific gravity of 1.98-2.20. Unlike common opal, precious fire opal displays exceptional clarity and exhibits strong chromatic dispersion, creating its characteristic fire-like appearance. The most valuable specimens combine high transparency with intense body color and strong play-of-color.
Precious fire is typically a natural gemstone.
Common names for Precious fire opal include cherry opal, girasol opal, Mexican opal, and sun opal.
Fire opal, like other opals, ranks about 5.5 to 6.5 on the Mohs scale of mineral hardness. This makes it relatively softer compared to many other gemstones, which means it can be more prone to scratching.
The refractive index of fire opal ranges from approximately 1.37 to 1.52. This RI contributes to the gemstone’s bright and vivid appearance, allowing it to capture light effectively.
Fire opal exhibits a subvitreous to waxy luster, which is typical of opal varieties. This luster gives the stone a beautiful glow that enhances its fiery coloration.
Fire opal, like other types of opal, does not have any cleavage. This means that it does not break along any defined plane when subjected to stress.
This gemstone typically exhibits a conchoidal fracture, which is a curved break similar to that seen in glass.
The specific gravity of fire opal ranges between about 2.15 and 2.21, which is considered typical for opals. This measure gives an idea of the density of the mineral compared to water.
Opals, including fire opal, are isotropic and do not show double refraction. They are single refractive due to their amorphous character.
Fire opal has relatively low dispersion when compared to other gemstones like diamond. However, its intense color can compensate by providing a different type of visual appeal.
Opals, including fire opal, are amorphous, meaning they do not have a crystalline structure. This is due to their formation from silica gel that does not crystallize upon hardening.
Fire opal is known for its vibrant orange, red, and yellow colors, which are where it gets its name. The colors can range from faint to vivid, often resembling the colors of a flame.
Fire opals can vary from opaque to translucent, with some high-quality stones being nearly transparent, which enhances their color brightness and depth.
Being isotropic, fire opals do not exhibit pleochroism. This means they show the same color in all directions under polarized light.
Fire opals typically do not exhibit fluorescence under UV light, unlike some other types of opal that may show a weak response.
Opals, including fire opals, generally have fair to good toughness, but care should still be taken as they can be damaged by impact due to their relatively low hardness.
Fire opal is brittle, as are most opals. This means that it can break or crack under stress or if knocked.
As isotropic gems, fire opals have an optic sign of “”+””, typical for materials that have the same optical properties in all directions.
Fire opal generally does not exhibit a characteristic absorption spectrum due to its amorphous nature and lack of significant impurities that affect light absorption.
Fire opal is made up of silicon dioxide (SiO2) with water content sometimes as high as 10%, which significantly influences its structure and physical properties.
While rare, some fire opals can show chatoyancy or the cat’s eye effect, when cut in a cabochon shape from material that displays aligned fibrous inclusions.
Asterism is not typically observed in fire opals. This effect is more commonly associated with other gemstones like star sapphires or star rubies.
Some fire opals can display iridescence, particularly those with a play-of-color, although this is less common than in other types of opals like the precious opal.
Fire opal is not magnetic. Its chemical composition does not include magnetic elements.
As a type of silica, fire opal is an insulator and does not conduct electricity effectively.
Fire opals are not radioactive and pose no health risks related to radioactivity.
