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Semiopal is a variety of common opal characterized by its translucent to opaque appearance and incomplete play of color, displaying partial iridescence across its surface. The mineral belongs to the silicate class, composed primarily of hydrated silica (SiO2·nH2O) with varying amounts of water content.
The stone forms through the deposition of silica-rich solutions in rock cavities at relatively low temperatures, creating a distinctive internal structure of microscopic silica spheres. These spheres, while present, are less uniformly arranged compared to precious opal, resulting in its partial or sectional color play. The stone typically exhibits a hardness of 5.5-6.5 on the Mohs scale and possesses a vitreous to resinous luster.
Semiopals occur in various geological environments, predominantly in volcanic regions and sedimentary deposits. The stone’s color spectrum includes white, yellow, brown, and gray base colors, with iridescent patches displaying blues, greens, and occasional reds. Major deposits exist in Australia, Mexico, and Brazil, where they form in association with other silica minerals.
Semiopal is typically a natural gemstone.
Common names for Semiopal opal include milk opal, jelly opal, and water opal.
Semiopal has a Mohs hardness of around 5.5 to 6.5, making it relatively softer than many other gemstones. This level of hardness means it can be somewhat susceptible to scratches and abrasion.
The refractive index of semiopal generally ranges from 1.42 to 1.45, which is relatively low compared to many other gemstones. This contributes to its distinctive, subtle luster.
Semiopal exhibits a waxy to slightly pearly luster, which gives it a soft glow as opposed to a brilliant sparkle.
Semiopal typically does not have any natural planes of cleavage, which is a characteristic common to all opals.
This gemstone usually exhibits a conchoidal to uneven fracture, which can make it challenging to cut and polish without chipping.
The specific gravity of semiopal fluctuates around 1.9 to 2.3, which is relatively low, indicating a less dense material compared to many other gemstones.
Semiopal is amorphous and does not display birefringence, as it lacks a crystalline structure.
Semiopal typically shows minimal dispersion or “”fire,”” especially when compared to its more colorful counterparts like precious opal.
Being an amorphous solid, semiopal does not belong to any crystal system, which differentiates it from crystalline gemstones.
The colors of semiopal can range widely, including white, gray, blue, green, and a mixture of these colors. It lacks the play-of-color seen in precious opals but may exhibit a uniform coloration.
Semiopal varies from opaque to translucent. The level of transparency often affects the perceived depth and allure of the stone’s color.
Semiopal does not exhibit pleochroism due to its amorphous nature and uniform internal structure.
Some semiopals may exhibit weak fluorescence under UV light, typically showing green or white colors.
Semiopal has a fair to good toughness, making it relatively durable against knocks and impacts compared to other more brittle gemstones.
The tenacity of semiopal is generally brittle, which coincides with its fracture characteristics and amorphous structure.
As an amorphous gemstone, semiopal has an isotropic optic sign.
Semiopal does not display a characteristic absorption spectrum due to its lack of a crystalline structure and uniformity in color.
Semiopal consists mainly of hydrated silicon dioxide (SiO2·nH2O), similar to other types of opal but with varying water content and impurities that affect its properties and appearance.
While not common, some semiopals can exhibit chatoyancy when cut appropriately, especially when they contain parallel inclusions or fibrous structures.
Asterism is not typically observed in semiopal due to its amorphous nature and lack of the necessary internal crystal structure.
Unlike precious opals, semiopal generally does not show iridescence. Any such effect would be subtle and not comparable to the vibrant play-of-color seen in its more famous counterparts.
Semiopal exhibits no magnetic properties due to its silicon dioxide composition, which is non-metallic.
Being a type of opal, semiopal is generally a poor conductor of electricity, consistent with its predominantly siliceous composition.
Semiopal does not exhibit any significant levels of radioactivity, making it safe for regular handling and wear.
