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Milk opal is a rare variety of common opal characterized by its opaque white body color and distinct milky appearance, composed primarily of hydrated silica (SiO2·nH2O) with microscopic spheres of amorphous silica creating its distinctive optical properties. This cryptocrystalline form occurs in volcanic regions where silica-rich solutions penetrate host rocks, typically forming in the cavities of basaltic formations.
The stone exhibits a unique internal structure of densely packed silica spheres measuring 150-300 nanometers in diameter, which causes Rayleigh scattering of light waves, producing its characteristic white color. Found predominantly in volcanic regions of Hungary, Mexico, and Oregon, milk opal forms through low-temperature hydrothermal processes at depths between 50-100 meters below surface level. Unlike precious opal, milk opal lacks play-of-color but maintains a consistent 6-6.5 Mohs hardness rating and specific gravity of 2.1-2.2.
Milk opal is typically a natural gemstone.
Common names for Milk opal include milky opal, white opal, and opalite, though it’s important to note that “opalite” can also refer to a man-made glass and not just the naturally occurring Milk opal variety.
Milk opal, a variety of common opal, generally has a hardness of about 5.5 to 6 on the Mohs scale. This makes it relatively softer compared to many other gemstones and somewhat susceptible to scratches and abrasion.
The refractive index of milk opal typically ranges from 1.44 to 1.46. This is relatively low compared to many other gemstones, contributing to its distinct optical properties.
Milk opal exhibits a waxy to pearly luster, which is characteristic of opal varieties. The presence of water within its structure gives it a unique sheen.
Milk opal does not have any natural planes of cleavage, which is a common feature in the opal family.
This gemstone typically displays a conchoidal to uneven fracture, making it brittle and sensitive to high impact or pressure.
The specific gravity of milk opal ranges approximately between 2.15 and 2.20, which is slightly higher than that of most other types of opal due to its relative lack of water content compared to other varieties.
Milk opal is isotropic and shows no double refraction, as it is amorphous in nature.
Milk opal generally shows low to moderate dispersion, also known as fire. This can vary based on the specific specimen and its inherent qualities.
Being an amorphous mineraloid, milk opal does not have a crystal system. Its structure lacks the long-range order typical of crystalline substances.
Milk opal is typically white or very light in color, with a milky appearance due to the diffusion of light by microscopic silica spheres within its matrix.
This type of opal ranges from translucent to opaque, which again contributes to its milky, soft appearance.
Milk opal does not exhibit pleochroism due to its amorphous nature and uniform internal structure.
Milk opal can sometimes show fluorescence under UV light, typically glowing green or white depending on the specific impurities within the stone.
Due to its amorphous and somewhat brittle nature, milk opal is considered to have fair to poor toughness.
Milk opal is generally brittle, which means it can easily crack or chip upon impact.
As an isotropic material, milk opal has no optic sign.
Typically, milk opal does not display a characteristic absorption spectrum due to its lack of crystal structure and uniform composition.
Milk opal is composed primarily of silica (SiO2) and water, with the water content sometimes reaching up to 20% by weight.
While rare in milk opal, chatoyancy can sometimes occur if the stone contains aligned fibrous inclusions.
Asterism is not typically observed in milk opal due to its uniform internal structure without the necessary aligned inclusions.
Milk opal may display a form of opalescence or a subdued play of color, which is not as pronounced as in precious opal but can still be visually appealing.
Milk opal is not magnetic, consistent with most other types of opal.
As a non-metallic mineral, milk opal is an insulator and does not conduct electricity effectively.
Milk opal is not radioactive and poses no health hazards in this regard.
