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Flash opal is a rare variety of precious opal characterized by a distinctive optical phenomenon where vibrant colors flash directionally across the stone’s surface when viewed from specific angles. The phenomenon occurs due to the diffraction of light through uniformly-sized silica spheres arranged in a regular pattern within the opal’s microstructure.
The most valuable flash opals exhibit a dominant flash color, typically red, blue, or green, that appears and disappears as the viewing angle changes. This directional color play, known as directional schiller, distinguishes flash opals from common opals that display non-directional play-of-color. The intensity and quality of the flash depend on the size, uniformity, and arrangement of the silica spheres, which typically measure between 150 to 300 nanometers in diameter.
These gemstones form in volcanic rocks where silica-rich solutions penetrate cavities and slowly crystallize under specific geological conditions. The finest specimens originate from Australia’s Lightning Ridge and Coober Pedy deposits, where they occur within ironstone concretions.
Flash Opal is typically a natural gemstone.
Common names for Flash Opal include Fire Opal, Girasol Opal, and Lightening Opal.
Flash 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 more susceptible to scratching and chipping compared to harder gemstones.
The refractive index of flash opal is typically in the range of 1.44 to 1.46. This property helps in determining how much the light is bent or refracted when entering the stone.
Flash opal exhibits a vitreous to pearly luster, contributing to its appealing visual effects and the way it interacts with light.
Opal, including flash opal, generally does not have cleavage. It breaks randomly, which is a characteristic important for cutters to consider during the gemstone’s processing.
Flash opal usually possesses a conchoidal to uneven fracture, which is typical of many types of opals.
The specific gravity of flash opal ranges from approximately 1.98 to 2.25.
Opal is amorphous and does not exhibit birefringence, meaning that it has the same refractive index in all directions.
Flash opal is noted for its remarkable play of color, or “”fire,”” which is not technically dispersion but rather the diffraction of light through its silica structure. Dispersion in the context of fire in gemstones is relatively low.
Being an amorphous mineraloid, flash opal does not have a crystal system. It lacks a crystalline structure, which is typical for true minerals.
Flash opal displays a wide range of colors, often showing brilliant flashes of various colors including green, blue, red, and yellow within a single stone.
This type of opal can range from opaque to translucent. The degree of transparency can affect the play of color and the overall appearance of the gemstone.
Flash opal does not display pleochroism due to its amorphous nature.
Some flash opals may exhibit fluorescence under UV light, typically showing green or white colors.
Opal is generally fragile due to its relatively high water content and low hardness. Flash opal’s toughness can be considered fair to poor.
Flash opal is somewhat brittle, which aligns with its molecular structure and composition.
Being isotropic, flash opal has an optic sign of “”none,”” reflecting its amorphous character.
Typically, flash opal does not exhibit a characteristic absorption spectrum due to its amorphous nature and varied composition.
Flash opal is primarily made of silica (SiO2·nH2O) and contains a significant amount of water (up to 20%).
While not common, some opals like flash opal may exhibit a cat’s eye effect if they have aligned fibrous inclusions.
Asterism or the star effect is not typically observed in flash opal.
The iridescence in flash opal, which is a type of opalescence, is due to the diffraction of light passing through tiny, regular spheres of silica gel within the stone.
Flash opal exhibits no magnetic properties.
As a non-metallic mineraloid, flash opal has poor electrical conductivity.
Flash opal typically does not display any significant levels of radioactivity.
