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Opal matrix is a naturally occurring host rock formation that contains precious opal deposits within its structural cavities and fissures. The matrix consists primarily of sedimentary or volcanic materials, most commonly sandstone, claystone, or ironstone, which serve as the foundational structure where opal forms through silica-rich water deposition.
Australian opal fields, particularly those in Coober Pedy and Lightning Ridge, produce distinctive matrix specimens where precious opal intrudes into the microscopic spaces of the host rock. The interaction between the opal and matrix creates unique patterns, with opal appearing as veins, spots, or replacing organic matter in fossilized remains. The matrix’s mineral composition influences the opal’s stability and preservation, while its porosity determines the distribution and quality of opal formation within the stone. Gemologists classify opal matrix specimens based on the percentage of precious opal present, pattern distribution, and host rock composition.
Opal matrix is typically a natural gemstone.
Common names for Opal matrix opal include Andamooka matrix opal, treated matrix opal, and Queensland matrix opal.
Opal matrix, being a composite of opal and its host rock, typically possesses a hardness of about 5.5 to 6.5 on the Mohs scale. This value can vary slightly depending on the type and proportion of host rock material intertwined with the opal.
Opal matrix has a refractive index generally ranging from 1.37 to 1.52. This range reflects variations in the opal and host rock compositions.
The luster of opal matrix can range from waxy to dull, depending on the opal’s quality and the type of host rock. The polished surfaces might exhibit more of an opal’s characteristic subvitreous (glass-like) luster.
Opal matrix typically exhibits no cleavage as opal itself is an amorphous mineraloid and does not have a crystalline structure necessary for cleavage planes.
This gemstone commonly displays a conchoidal to uneven fracture, which is typical of opal within its matrix.
The specific gravity of opal matrix can vary significantly, generally between 2.15 and 2.20, influenced by the density of the incorporated host rock.
Opal matrix exhibits no double refraction due to opal being isotropic, lacking the crystalline structure required for birefringence.
Opal matrix can show a variable degree of dispersion or “”fire,”” which is the play of color due to diffraction of light. The intensity and quality of fire depend on the quality and type of opal present in the matrix.
Being composed partly of amorphous opal, the opal matrix does not have a crystal system. The host rock may have its own distinct crystalline structure, but this does not apply to the opal itself.
The color of opal matrix can vary widely, incorporating the broad color play of opal—ranging from white and gray to black, with possible flashes of various colors—combined with the color of the host rock.
Opal matrix ranges from opaque to slightly translucent, depending on the thickness and composition of the opal and host rock.
Opal matrix does not exhibit pleochroism due to the isotropic nature of opal.
Opal matrix might fluoresce under UV light depending on the type of opal and its impurities, typically showing green, white, or no fluorescence at all.
The toughness of opal matrix is generally fair to good, which makes it somewhat sensitive to impact and pressure.
Opal matrix is brittle, like pure opal, and can be prone to cracking or chipping upon impact.
As an isotropic substance, opal shows an optic sign of being uniaxial, which in practical terms for opal matrix means there is no variation in optical properties with direction.
The absorption spectrum of opal matrix does not typically show distinctive features unless specific trace elements are present, which can sometimes absorb particular wavelengths of light.
Opal matrix consists of hydrated silica (SiO2·nH2O) with various inclusions of the host rock, which can include a wide range of minerals depending on the geological environment.
Opal matrix may show chatoyancy if it contains aligned fibers of another mineral or has internal structures that reflect light in a slit-like pattern.
While rare, some types of opal matrix might exhibit asterism if included crystals or fibers are arranged in a certain way to reflect light in a star pattern.
One of the most desirable features of some opal matrices is their iridescence, where the material shows shifting colors depending on the angle of light.
Opal matrix is generally non-magnetic, although magnetic properties can be introduced if the host rock contains magnetic minerals.
Opal matrix is typically an insulator due to its major component, opal, being composed of silica.
Typically, opal matrix is not radioactive. However, the presence of radioactive minerals in the host rock can impart some level of radioactivity to the matrix.
