Common Opal
Opal (Amorphous Hydrous Silica) · Semi-Precious Gemstone / Mineral Specimen
Crystal System
Amorphous (Non-crystalline); typically occurs in massive, botryoidal, or reniform habits
Mineral Group
Mineraloid (Silica group)
Chemical Formula
SiO2·nH2O (hydrated silica)
Formation Process
Sedimentary or Hydrothermal: Formed by the precipitation of silica-rich water in cavities and cracks at relatively low temperatures
About This Crystal
A translucent to opaque specimen with a waxy to vitreous luster. This specific piece shows a creamy yellowish-white body color with a bumpy, potch-like surface texture and a visible host rock matrix attachment.
Physical Characteristics
Color: Cream, white, or pale yellow; Streak: White; Cleavage: None; Fracture: Conchoidal to uneven; Specific Gravity: 1.9-2.3; Tenacity: Brittle; Fluorescence: Can show white, green, or blue under UV.
Optical Properties
Refractive Index: 1.37-1.47 (Isotropic); Birefringence: None; lacks the 'play-of-color' seen in precious opal; body color is caused by light scattering and trace impurities.
Hardness & Durability
Origin Region
Notably Australia (Coober Pedy), Ethiopia (Welo), Mexico, and the USA (Oregon/Nevada); often found in volcanic cavities or sedimentary layers
Hardness & Durability
Mohs Hardness: 5.5 to 6.5; Toughness: Very poor to fair; highly sensitive to thermal shock and dehydration.
Care & Maintenance
Clean only with warm soapy water; avoid ultrasonic or steam cleaners; store in a cool, humid environment to prevent 'crazing' (cracking) from water loss; keep away from harsh chemicals.
Rarity & Value
Common; low market value compared to precious opal; value is primarily for lapidary rough or beginner mineral collections ($1-$10 for small rough specimens).
Special Characteristics
Amorphous structure containing 3% to 21% water; this specimen lacks the diffraction grating structure required for opalescence/play-of-color.
Lore & History
Historically associated with luck and protection; in metaphysical traditions, white/common opal is believed to promote emotional balance and soothe the crown chakra.