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Milky opal is a variety of common opal characterized by its translucent to opaque appearance with a distinct white or bluish-white body color, lacking the play-of-color found in precious opal. The stone’s milky appearance results from microscopic silica spheres arranged in an irregular pattern, causing light to scatter diffusely through the material.
The mineralogical composition consists of hydrated silica (SiO2¬∑nH2O) with water content varying between 6-10%. This gem forms in volcanic rocks where silica-rich solutions penetrate cavities and fissures, depositing opal at low temperatures. The white coloration stems from the internal structure’s complete scattering of visible light wavelengths.
Found predominantly in deposits across Australia, Brazil, and Mexico, milky opal often occurs alongside other opal varieties in volcanic host rocks. The stone exhibits a vitreous to waxy luster, Mohs hardness of 5.5-6.0, and a specific gravity ranging from 1.98-2.20. Distinguished from other opal varieties by its uniform whiteness and lack of iridescence, milky opal remains valued in jewelry and ornamental applications.
Milky opal is typically a natural gemstone.
Common names for Milky Opal include White Opal, Milk Opal, and Opalite, though it’s important to note that “Opalite” can also refer to a man-made glass and not just the natural stone.
Milky Opal has a hardness of about 5.5 to 6.5 on the Mohs scale, indicating it has moderate scratch resistance.
The refractive index of Milky Opal ranges from 1.44 to 1.46, which is relatively low compared to many other gemstones.
Milky Opal exhibits a subdued, pearly luster due to its diffuse reflection of light.
Opal, including Milky Opal, does not have any cleavage. It is an amorphous substance.
Milky Opal typically has a conchoidal to uneven fracture, which is characteristic of many types of opal.
The specific gravity of Milky Opal usually falls between 1.98 and 2.25.
Being an amorphous gemstone, Milky Opal shows no birefringence.
Milky Opal typically exhibits low to moderate dispersion, causing fewer and softer flashes of spectral colors compared to other more prismatic gems.
Opal, including Milky Opal, is amorphous and does not have a crystal system.
Milky Opal is generally white or very light in color with a translucent to opaque transparency. It may include flashes of color depending on the presence of precious opal within the stone.
This gemstone is mostly opaque to translucent.
Milky Opal does not exhibit pleochroism due to its amorphous structure.
Milky Opal can sometimes show a weak green or white fluorescence under ultraviolet light; however, this is not always the case.
Opal is generally fragile due to its high water content and internal structure, making it sensitive to heat and pressure.
Milky Opal is brittle, a common trait for opals due to their internal microstructure and water content.
Being isotropic, Milky Opal does not have an optic sign.
Milky Opal typically does not show a distinctive absorption spectrum.
Milky Opal is a hydrated form of silica (SiO2·nH2O).
Milky Opal rarely shows chatoyancy unless it has inclusions that might cause this effect.
Asterism is not typically observed in Milky Opal.
The presence of iridescence in Milky Opal depends on the formation of precious opal within the stone, which can display a play-of-color.
Milky Opal is not magnetic.
As a form of silica, Milky Opal has poor electrical conductivity.
Milky Opal is not radioactive.
