Boulder Opal
Amorphous Hydrated Silica with Ironstone · Semi-Precious Gemstone / Mineral Specimen
Crystal System
Amorphous (Non-crystalline); occurs as veins and infills within host rock
Mineral Group
Silicates (Tectosilicate)
Chemical Formula
SiO2·nH2O + Fe2O3 (Ironstone Matrix)
Formation Process
Sedimentary. Silica-rich water seeps into cracks and voids in ironstone boulders, precipitating over millions of years.
About This Crystal
A dark brown, earthy ironstone matrix containing thin veins of precious opal. The surface shows a wet, resinous to vitreous luster where the opal is exposed, displaying flashes of blue, green, and gold against a dark background.
Physical Characteristics
Color: Brown matrix with multicolored opal; Streak: White; Cleavage: None; Fracture: Conchoidal to uneven; Specific Gravity: 2.1-2.5; Fluorescence: Inert to weak green/white.
Optical Properties
Refractive Index: 1.37-1.47; Single refraction; Displays 'Play of Color' (diffraction of light through silica spheres).
Hardness & Durability
Origin Region
Queensland, Australia (Winton, Quilpie, Yowah regions)
Hardness & Durability
Mohs Hardness: 5.5-6.5; Brittle. The ironstone backing provides better durability than solid opal, making it suitable for protected jewelry settings.
Care & Maintenance
Avoid ultrasonic cleaners and steam. Clean with warm soapy water. Do not expose to extreme temperature changes or harsh chemicals. Store away from direct sunlight to prevent dehydration.
Rarity & Value
Uncommon; Value is determined by the intensity and 'fire' of the opal veins against the matrix. Specimens with vibrant red or multi-color flashes are most prized.
Special Characteristics
Displays Play of Color; naturally occurring composite where the precious opal is host-bound to the ironstone boulder.
Lore & History
Discovered in the late 19th century in Queensland. Symbolically associated with purity, emotional intensification, and creativity. Traditionally used by indigenous Australians in Dreamtime legends.