Common Opal
Amorphous Hydrous Silica · Semi-Precious Gemstone / Mineral Specimen
Crystal System
Amorphous (Non-crystalline); typically occurs in massive, botryoidal, or stalactitic habits.
Mineral Group
Silicates (Tectosilicate class, though technically a mineraloid)
Chemical Formula
SiO2·nH2O
Formation Process
Sedimentary/Hydrothermal: Formed from silica-rich water seeping into cavities and fractures, followed by slow evaporation and solidification at low temperatures.
About This Crystal
This specimen is a yellowish-tan to olive-green common opal. It is translucent to opaque with a characteristic waxy to resinous luster and lacks the 'play of color' found in precious opal. It displays smooth, rounded surfaces consistent with its amorphous nature.
Physical Characteristics
Color: Yellow-khaki; Streak: White; Cleavage: None; Fracture: Conchoidal (shell-like); Specific Gravity: 1.9–2.3; Fluorescence: Often glows green or white under UV light.
Optical Properties
Isotropic (Singly refractive); Refractive Index: 1.37–1.47; No birefringence or pleochroism. Lacks the diffraction-grating effect of precious opal.
Hardness & Durability
Origin Region
Prominent sources include Australia, Ethiopia, Mexico, and the USA (Oregon/Idaho). Found in volcanic rocks (rhyolite) and sedimentary basins.
Hardness & Durability
Mohs Hardness: 5.5–6.5. It is brittle and sensitive to thermal shock and pressure. Lower durability than quartz.
Care & Maintenance
Avoid ultrasonic cleaners and steam. Clean with warm soapy water. Protect from extreme temperature changes and direct sunlight to prevent 'crazing' (cracking) due to water loss.
Rarity & Value
Common and abundant. Value is generally low for rough specimens, determined by color saturation and size. Rarely treated, though sometimes dyed.
Special Characteristics
Displays 'opalescence' (a milky sheen) and conchoidal fracturing. This specific specimen appears to be 'Potch' or common yellow opal.
Lore & History
Historically used for decorative carvings. In metaphysical traditions, common opal is associated with emotional grounding, the solar plexus chakra, and is believed to promote calm and centeredness.