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Mexican fire opal is a transparent to translucent variety of opal (SiO2·nH2O) characterized by its intense red, orange, and yellow play-of-color, formed through hydrothermal processes in the rhyolitic lava fields of Mexico. This rare gemstone contains between 3-10% water content within its amorphous silica structure, distinguishing it from common opal by its vitreous luster and higher refractive index.
The gem forms in volcanic rock cavities where silicon-rich water deposits crystallize at temperatures between 100-300 degrees Celsius. Its distinctive fire-like hues result from trace elements including iron and magnesium, while its structural stability surpasses that of other opal varieties. The finest specimens originate from the Mexican state of Querétaro, particularly the mines near Magdalena and San Juan del Río, where the volcanic environment creates ideal formation conditions. The stone exhibits a hardness of 6-6.5 on the Mohs scale and displays unique optical phenomena including schiller and contra luz effects when properly cut.
Mexican fire opal is typically a natural gemstone.
Common names for Mexican fire opal include cherry opal, girasol opal, and sun opal.
Mexican Fire Opal has a Mohs hardness of 5.5 to 6.5, making it relatively soft compared to many other gemstones. This means it’s more susceptible to scratches and should be worn carefully to avoid damage.
The refractive index of Mexican Fire Opal ranges from 1.430 to 1.460, which contributes to its bright and vivid appearance, although it’s not as high as some other gemstones like diamonds.
This gemstone typically exhibits a subvitreous to waxy luster, giving it a somewhat glossy look that is especially noticeable when the stone is well-polished.
Mexican Fire Opal does not have any natural planes of cleavage, which means it does not split easily along specific flat surfaces.
The typical fracture in a Mexican Fire Opal is conchoidal (shell-like), which is common among amorphous and cryptocrystalline minerals.
The specific gravity of Mexican Fire Opal usually ranges between 2.10 and 2.20, which is relatively low, indicating that the stone is not very dense.
Mexican Fire Opal exhibits no double refraction as it is isotropic, being amorphous in structure.
The dispersion of Mexican Fire Opal is generally low, which means it shows less of the rainbow-like color play seen in gemstones with higher dispersion.
Mexican Fire Opal is amorphous, meaning it does not have a regular crystal structure. This is typical for opals, which are formed from silica gel.
The color of Mexican Fire Opal can range from clear through yellow, orange, and red. Fire opals are particularly known for their bright, fiery orange color, sometimes with a rich red hue.
Mexican Fire Opals can range from transparent to translucent, with the transparent varieties being more highly valued.
Pleochroism is not observed in Mexican Fire Opal due to its isotropic nature.
Mexican Fire Opal may show green or brownish-green fluorescence under UV light, although this is not particularly strong or notable in all specimens.
The toughness of Mexican Fire Opal is generally fair, but the stone can be brittle, especially along edges or in thinner pieces.
Mexican Fire Opal is brittle, which makes it more prone to cracking under stress.
Being isotropic, Mexican Fire Opal has an optic sign of “”isotropic,”” showing uniform optical properties in all directions.
Mexican Fire Opal does not typically show a distinct absorption spectrum due to its amorphous nature and lack of strong pleochroism.
Mexican Fire Opal is composed of SiO2·nH2O (silicon dioxide with water content), which is characteristic of all opals.
While not common, some Mexican Fire Opals can exhibit a cat’s eye effect when cut en cabochon and aligned properly with the internal structure of the stone.
Asterism or the star effect is not typically seen in Mexican Fire Opal.
Some varieties of Mexican Fire Opal may display iridescence, particularly those with a significant amount of internal structure, such as inclusions or layering.
Mexican Fire Opal is not magnetic.
Being a type of silica gel, Mexican Fire Opal has low electrical conductivity.
Mexican Fire Opal is not radioactive.
