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White opal is a hydrated amorphous form of silica (SiO2·nH2O) characterized by its distinctive white body color and iridescent play-of-color, displaying spectral hues through a phenomenon called diffraction. The gemstone forms in sedimentary rocks when silica-rich water fills cracks and cavities, creating a precise arrangement of microscopic silica spheres.
The unique internal structure consists of regularly arranged spheres of silica approximately 150-300 nanometers in diameter, which creates a three-dimensional diffraction grating. These spheres diffract white light into its spectral components, producing the characteristic opalescence. White opal exhibits a potch (common opal) base with a milky to snow-white background, against which the play-of-color appears most vivid.
This variety ranks 5.5-6.5 on the Mohs hardness scale and contains 6-10% water by weight. The finest specimens originate from Australia’s Great Artesian Basin, particularly the Lightning Ridge and Coober Pedy regions, where they formed during the Cretaceous period approximately 100 million years ago.
White opal is typically a natural gemstone.
Common names for White Opal include Milk Opal, White Milk Opal, and Silky Opal.
White opal has a hardness of approximately 5.5 – 6.5 on the Mohs scale, which makes it relatively softer than many other gemstones. This level of hardness indicates that while it is somewhat durable, it requires careful handling to avoid scratches and damage.
The refractive index of white opal ranges from about 1.44 to 1.46, which is relatively low compared to many other gemstones. This contributes to its unique play of color.
White opal exhibits a subvitreous to waxy luster, which gives it a soft and appealing glow. Its luster is one of the key characteristics that contribute to its beauty.
Opal, including white opal, generally does not have any cleavage. This means it does not break along well-defined planes.
White opal typically displays a conchoidal or uneven fracture. This type of fracture can result in a smooth, curved surface when the stone is broken.
The specific gravity of white opal ranges from approximately 2.15 to 2.21. This measurement indicates that opal is relatively light compared to many other gemstones.
Opal, including white opal, is isotropic and exhibits no double refraction. This means that it has the same optical properties in all directions.
White opal can display a notable level of dispersion or “”fire,”” which is the play of rainbow-colored light across the stone. This effect is particularly valued in opal.
White opal is amorphous, meaning it does not have a crystalline structure. This distinguishes it from crystalline gemstones.
White opal features a light body tone ranging from colorless to milky white, which can exhibit a play of color in bright, iridescent flashes across its surface.
The transparency of white opal can vary from opaque to translucent. Translucent specimens are typically more valued because they better display opal’s characteristic play of color.
White opal does not exhibit pleochroism due to its isotropic, amorphous nature.
White opal can sometimes show fluorescence under ultraviolet light, typically displaying a weak green or white color.
Opal has fair to good toughness, but it can be brittle and sensitive to changes in temperature and humidity, which can cause cracking.
White opal is generally brittle, meaning it can easily break or chip upon impact or under stress.
Being isotropic, white opal has an optic sign of “”isotropic,”” showing uniform properties in all directions of light.
White opal does not typically show a distinctive absorption spectrum, unlike some other colored gemstones.
White opal is a hydrated form of silica (SiO2·nH2O). Its water content can range from 3 to 21% by weight, which affects its physical properties.
While typically associated with other varieties of opal, chatoyancy or the cat’s eye effect can occasionally be seen in white opals if they have columnar structures within.
Asterism or the star effect is not commonly observed in white opals.
One of the most appealing features of white opal is its iridescence, presenting a moving play of rainbow colors, often visible across the surface of the gemstone.
White opal is not magnetic.
As a form of silica, white opal is a poor conductor of electricity.
White opal is not radioactive.
