Opal
Hydrated Amorphous Silica var. Common Opal · Semi-Precious Gemstone
Crystal System
Amorphous (Non-crystalline)
Mineral Group
Silicates
Chemical Formula
SiO2·nH2O
Formation Process
Sedimentary and weathering processes involving silica-rich water filling fissures and cavities at low temperatures
About This Crystal
An oval-shaped, smooth, tumbled specimen exhibiting a pale greenish-white to off-white body color, translucent transparency, and a waxy to vitreous luster typical of common opal.
Physical Characteristics
Pale green/white body color, white streak, no cleavage, conchoidal fracture, specific gravity around 2.10 to 2.20, brittle tenacity, inert to moderate fluorescence depending on locality
Optical Properties
Refractive index ranges from 1.370 to 1.470, singly refractive (isotropic due to amorphous structure), lacks the play-of-color seen in precious opal
Hardness & Durability
Origin Region
Australia, Mexico, Ethiopia, United States
Hardness & Durability
Mohs hardness of 5.5 to 6.5, poor to fair toughness, sensitive to rapid temperature changes, dehydration, and strong acids; moderate wearability for jewelry
Care & Maintenance
Clean with warm soapy water using a soft cloth; avoid ultrasonic cleaners, steam, harsh chemicals, and prolonged exposure to direct sunlight or extreme dryness which can cause cracking
Rarity & Value
Common opal is widespread and generally affordable, with value determined by size, cut quality, and body color consistency rather than play-of-color
Special Characteristics
Amorphous internal structure lacking a true crystal lattice; some varieties may exhibit hyalite-like green fluorescence under UV light
Lore & History
Historically associated with hope, innocence, and emotional healing; in crystal lore, it is believed to enhance personal transformation and soothe emotional states