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Fire opal is a translucent to transparent variety of opal displaying warm body colors of yellow, orange, or red, caused by traces of iron oxide within its chemical structure of hydrated silica (SiO2·nH2O). The gem occurs primarily in volcanic regions, formed when silica-rich water fills cavities in rhyolite and other igneous rocks.
Mexican fire opals, particularly from the state of Querétaro, represent the finest known specimens, exhibiting intense play-of-color phenomena alongside their characteristic body color. These gems form in gas cavities of volcanic rocks at temperatures between 100-300 degrees Celsius, creating distinctive internal structures that produce their vivid hues. The most valuable specimens display high transparency, intense orange-red coloration, and minimal inclusions.
The stone’s microstructure consists of spheres of silica arranged in a regular pattern, with sizes ranging from 150-300 nanometers, determining both its color and optical properties. Unlike precious opal, fire opal rarely exhibits play-of-color, instead deriving its value from its body color’s intensity and clarity.
Fire Opal is typically a natural gemstone.
Common names for Fire Opal include Cherry Opal, Girasol Opal, Harlequin Opal, and Mexican Opal.
Fire opal, like other opals, has a relatively low hardness rating on the Mohs scale, typically ranging from 5.5 to 6.5. This makes it softer than many other gemstones, meaning it requires careful handling to prevent scratching or damage.
The refractive index of fire opal ranges between 1.450 and 1.470, which is generally lower than many other gemstones. This contributes to its unique visual properties.
Fire opal exhibits a vitreous to sub-vitreous luster when polished, giving it a glass-like surface that enhances its color and brightness.
Opal, including fire opal, does not have a true cleavage but may exhibit a conchoidal fracture, which can make it challenging to cut and shape.
Fire opal typically has a conchoidal fracture, which is a curved break similar to the way glass breaks. This type of fracture can give the stone a slightly uneven surface if it’s broken.
The specific gravity of fire opal ranges approximately from 2.10 to 2.20, which is somewhat lighter compared to many other gemstones.
Fire opal, like other types of opal, is amorphous and does not exhibit double refraction due to its lack of a crystalline structure.
The dispersion of fire opal is relatively low, which means it typically does not display the same type of fiery rainbow colors that you would see in diamond or other high-dispersion stones.
Opals, including fire opal, are amorphous, meaning they do not have a regular crystal structure. This is one of the key characteristics that define the opal family.
Fire opal is known for its warm body colors in hues of yellow, orange, and red, often vivid and intense, resembling the colors of a fiery sunset.
Fire opals can range from transparent to translucent, with the more transparent varieties being more highly valued.
Fire opal typically does not exhibit pleochroism due to its amorphous nature and uniform color distribution.
Fire opal may show weak green or brown fluorescence under UV light, but this is not a prominent feature of the stone.
Opal has fair to good toughness, but its relatively low hardness and lack of cleavage make it prone to scratching and chipping. Fire opal is no exception.
Fire opal’s tenacity is generally brittle due to its composition and structure, which means it can break under stress.
Being amorphous, fire opal does not have an optic sign as is typical with isotropic or anisotropic minerals.
Fire opal typically does not display a characteristic absorption spectrum due to its amorphous nature and lack of specific trace elements that absorb light.
Fire opal is a form of silica and primarily consists of silicon dioxide (SiO2) with water content that can range from 3% to 21%.
While not common, some fire opals can display chatoyancy or cat’s eye effect when cut as cabochons and aligned properly with the internal structure.
Asterism, or the star effect, is rare in fire opals and is generally not a feature associated with this type of gemstone.
Fire opal does not typically show iridescence, unlike some other forms of opal that display this phenomenon due to their internal structure.
Fire opal is not magnetic due to its chemical composition primarily being silicon dioxide.
Being a non-metallic mineral, fire opal has poor electrical conductivity.
Fire opal is not radioactive and poses no radioactive hazards.
