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Flame opal is a rare variety of precious opal characterized by its distinctive play-of-color featuring dominant red, orange, and yellow hues that create a flickering flame-like appearance within the gemstone’s transparent to translucent body.
The gem forms in volcanic regions when silica-rich solutions infiltrate rock cavities, depositing hydrated silica spheres in a regular pattern. These spheres, ranging from 150 to 300 nanometers in diameter, diffract light through a phenomenon known as Bragg diffraction, creating the opal’s signature fire effect. Mexican flame opals, particularly those from the Quer√©taro mines, exhibit the most intense color combinations and patterns. The stone’s refractive index typically ranges from 1.37 to 1.43, with a hardness of 5.5-6.5 on the Mohs scale. Unlike common opals, flame opals can display their characteristic fire without requiring a dark body tone, making them unique among precious opals.
Flame Opal is typically a natural gemstone.
Common names for Flame Opal include Fire Opal, Cherry Opal, Mexican Opal, and Sun Opal.
Flame opal, like other varieties of opal, has a relatively low hardness on the Mohs scale, typically ranging from 5.5 to 6.5. This makes it somewhat susceptible to scratching and requires careful handling and setting in jewelry to avoid damage.
The refractive index of flame opal typically ranges from about 1.44 to 1.46, which is relatively low compared to many other gemstones. This property contributes to its distinctive play of color.
Flame opal has a subvitreous to waxy luster, which gives it a soft glow that enhances its color play.
Opal, including flame opal, does not exhibit any cleavage, which is a break along crystallographic structural planes. This characteristic makes it relatively stable when being cut, although it is still a fragile material.
Flame opal typically has a conchoidal fracture, creating smooth, curved surfaces when broken. This type of fracture is common in more brittle materials.
The specific gravity of flame opal ranges approximately from 2.15 to 2.21, which is considered low and helps in identifying the gemstone through density tests.
Opals, including flame opal, are isotropic and therefore do not exhibit double refraction, meaning they have a single refractive index.
Although flame opal does not have significant dispersion, the play of color intrinsic to opals is similar to dispersion effects in other gemstones, resulting from its internal structure diffracting light into various colors.
Flame opal is amorphous, which means it lacks a crystalline structure. This attribute leads to the unique play of color seen in opals.
Flame opal displays a background color in fiery orange, red, or yellow hues, often with brilliant flashes of green, blue, and other colors depending on the angle of light and viewpoint.
Flame opals can range from opaque to translucent. The level of transparency can affect the vibrancy of the play of color.
As an isotropic material, flame opal does not exhibit pleochroism. This means the stone shows the same color in all directions.
Some flame opals may show fluorescence under ultraviolet light, typically displaying a greenish or whitish glow, but this can vary.
Opal is generally considered to have fair to good toughness, but its water content and internal structure can make it prone to drying and cracking.
Flame opal is relatively brittle, which makes it sensitive to being struck or stressed.
Being isotropic, flame opal has an optic sign of “”isotropic,”” showing uniform optical properties in all directions.
Flame opal generally does not display a characteristic absorption spectrum due to its amorphous nature.
Flame opal is a form of silica (SiO2·nH2O) and contains a varying amount of water, typically between 3% and 10%.
While more common in other gemstones, some opals including flame opal may exhibit a cat’s eye effect if they have aligned fibrous inclusions.
Asterism is not typically observed in flame opals. This effect is more common in other types of gemstones with suitable crystal structures and inclusions.
The defining characteristic of opals, including flame opal, is their iridescence or play of color, which is caused by the diffraction of light by the microscopic silica spheres within their structure.
Flame opal is not magnetic, which is consistent with its silica composition.
As a variety of opal, flame opal is an insulator and does not conduct electricity efficiently.
Flame opal is not radioactive and poses no risks of radioactivity to its wearers.
