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Seam opal is a distinct variety of precious opal found in horizontal or vertical fissures within host rock matrices, characterized by its formation in thin layers or veins. These opals exhibit play-of-color through a natural phenomenon called diffraction, where light interacts with microscopic silica spheres arranged in a regular pattern.
Australian seam opals, predominantly mined in Lightning Ridge and Coober Pedy, form in joints and fractures of sedimentary rocks, particularly mudstone and sandstone. The opal-bearing seams range from microscopic thickness to several centimeters, often displaying vivid spectral colors against dark body tones. These gems contain 6-10% water content within their amorphous silica structure, crystallizing at low temperatures between 20-35°C. The finest specimens showcase directional color patterns, with some rare pieces exhibiting columnar structure perpendicular to the seam walls.
Seam opal is typically a natural gemstone.
Common names for Seam opal include Andamooka Matrix opal, Boulder opal, and Matrix opal.
Seam opal typically has a hardness of 5.5 to 6.5 on the Mohs scale. This makes it relatively softer compared to many other popular gemstones, which means it can be more susceptible to scratches and abrasion.
The refractive index of seam opal ranges from about 1.37 to 1.52. This variation in RI contributes to the stone’s notable play of color.
Seam opal exhibits a subvitreous to waxy luster, which gives it a less shiny and more subdued appearance compared to gemstones with a vitreous (glass-like) luster.
Opal, including seam opal, typically does not exhibit any cleavage. This means that it does not break along defined planes.
Seam opal is characterized by a conchoidal to uneven fracture, which is typical of many types of opal.
The specific gravity of seam opal ranges approximately from 1.98 to 2.25. This is relatively low compared to many other gemstones, making opal lighter for its size.
Opal, including seam opal, is isotropic and shows no double refraction. This is due to its amorphous character.
Seam opal can display a noticeable level of dispersion or “”fire,”” which refers to the stone’s ability to disperse light into the colors of the spectrum. This quality can vary significantly based on the individual stone and its conditions of formation.
Opal, including seam opal, is amorphous and does not have a crystal system. This means it lacks a regular atomic structure that defines crystalline materials.
Seam opal can display a wide range of colors, often showing vibrant play of colors. The background color can be white, black, or any variant in between, influenced by the conditions under which the opal formed.
Seam opal ranges from opaque to translucent. This variation affects the visual appearance and the play of color of the gemstone.
Pleochroism is not observed in seam opal due to its isotropic nature.
Opals, including seam opal, may exhibit fluorescence under ultraviolet light, typically showing a white or creamy glow, though this can vary.
Opal’s toughness is generally fair to poor due to its natural brittleness and high water content, making it susceptible to cracking and chipping.
Seam opal is generally brittle, which is typical of opal due to its composition and structure.
Being isotropic, seam opal has no optic sign.
Seam opal generally does not exhibit a diagnostic absorption spectrum due to its amorphous nature.
Seam opal primarily consists of silicon dioxide (SiO2) and variable amounts of water. This composition can influence its physical properties and appearance.
While less common, some seam opals can exhibit chatoyancy, or the cat’s eye effect, when cut appropriately and when fibrous inclusions are present.
Asterism or the star effect is generally not observed in seam opal.
Seam opal is well known for its iridescence, particularly the play of color seen as the stone is moved under a light source.
Opal is not magnetic.
Opal, including seam opal, is generally a poor conductor of electricity due to its composition and structure.
Seam opal is not radioactive and presents no health hazards in this regard.
