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Dendritic opal is a form of common opal containing tree-like or branching inclusions of manganese or iron oxides, creating distinctive patterns reminiscent of organic plant growth. The mineral belongs to the opal-A group with a chemical composition of SiO2·nH2O, displaying no play-of-color characteristic.
The dendrites form through mineral infiltration during the opal’s formation process, when manganese-rich solutions penetrate the silica gel before complete solidification. These black or brown arborescent patterns range from microscopic to clearly visible structures, often resembling ferns, moss, or winter frost patterns on glass.
This variety of opal occurs primarily in deposits across Australia, Brazil, and Kazakhstan, forming in volcanic and sedimentary environments. The stone exhibits a Mohs hardness of 5.5-6.5 and possesses a vitreous to resinous luster. Unlike precious opal, dendritic opal maintains consistent optical properties without displaying opalescence, though it may present in various background colors from white to yellow.
Dendritic Opal is typically a natural gemstone.
Common names for Dendritic Opal include Merlinite, Moss Opal, and Tree Agate.
Dendritic opal, like other opals, generally has a hardness rating of about 5.5 to 6.5 on the Mohs scale. This makes it relatively softer compared to many other gemstones, which means it can be susceptible to scratches and abrasion.
The refractive index of dendritic opal ranges from about 1.44 to 1.46, which is typical for opals. This RI contributes to the overall play of color characteristic of opals.
Dendritic opal exhibits a waxy to dull luster, typical of opals. The luster can vary slightly depending on the quality of the polish and the presence of inclusions.
Opals, including dendritic opal, do not have a true cleavage. They are generally amorphous in structure, which means they lack a systematic plane of weakness along which they can be split.
Dendritic opal typically has a conchoidal to uneven fracture, which is a common feature in amorphous minerals.
The specific gravity of dendritic opal typically ranges from about 1.98 to 2.25. This can vary slightly depending on the amount and type of inclusions and their distribution within the stone.
Dendritic opal is isotropic, meaning it shows no double refraction when viewed under polarized light, consistent with its amorphous nature.
Opals, including dendritic opal, are known for their play of color rather than dispersion or fire. The dispersion is generally low.
Dendritic opal is amorphous, meaning it does not have a crystal system. It does not form crystals but instead consists of silica spheres.
Dendritic opal can range in color from colorless to white, with distinctive green or black dendritic (tree-like) inclusions of various manganese oxides or silicates.
This gemstone can range from opaque to slightly translucent, depending on the density and type of dendritic inclusions.
Dendritic opal shows no pleochroism due to its isotropic and amorphous nature.
Dendritic opal typically does not exhibit fluorescence under UV light, although there can be exceptions depending on specific impurities within the stone.
Opal is generally considered to have fair to good toughness, but this can be compromised by its natural inclusions, internal stresses, and any existing micro-cracks.
Dendritic opal is brittle, like other opals, which makes it sensitive to being hit or dropped.
Being isotropic, dendritic opal does not have an optic sign.
Dendritic opal does not typically display a characteristic absorption spectrum, owing to its amorphous nature and general lack of color-causing elements.
The chemical formula for dendritic opal is SiO₂·nH₂O; it’s a hydrated form of silica. The water content can range from 3% to 21%.
Dendritic opal generally does not exhibit chatoyancy, as this effect is more common in minerals with fibrous inclusions and a more defined crystal structure.
Asterism is not typically observed in dendritic opal due to its amorphous structure and lack of aligned inclusions necessary to produce the star effect.
While traditional opals are known for their play of color, dendritic opals generally do not show iridescence, though they may display interesting patterns due to their dendritic inclusions.
Dendritic opal is not magnetic, although the metallic inclusions that form the dendrites might show some magnetic properties depending on their composition.
As a type of opal, dendritic opal is an insulator and does not conduct electricity.
Dendritic opal is not radioactive and poses no health hazards in this regard.
