Boulder Opal
Amorphous Hydrated Silica (Opal) in Ironstone Matrix · Precious Gemstone / Mineral Specimen
Crystal System
Amorphous (Non-crystalline); host rock is typically massive Ironstone
Mineral Group
Silicates (Tectosilicates); Mineraloid Class
Chemical Formula
SiO2·nH2O (Silicon Dioxide with water) + Fe2O3 (Ironstone host)
Formation Process
Sedimentary/Hydrothermal; silica-rich water seeps into cracks in ironstone/sandstone over millions of years
About This Crystal
Opaque dark brown ironstone host with thin, vibrant veins of blue precious opal; vitreous to waxy luster on opal surfaces
Physical Characteristics
Dark brown body with blue streaks; white streak; conchoidal fracture in opal; specific gravity 2.1-2.5
Optical Properties
Isotropic; Play-of-color (blue/cyan) due to diffraction of light by silica spheres; non-refractive
Hardness & Durability
Origin Region
Queensland, Australia (Winton, Quilpie, Yowah fields)
Hardness & Durability
5.5 - 6.5 on Mohs scale; brittle; host rock provides better durability than solid opal
Care & Maintenance
Avoid chemicals, ultrasonic cleaners, and sudden temperature changes; use warm soapy water and soft cloth
Rarity & Value
Uncommon; value depends on intensity of color flashes and pattern; often more affordable than black opal
Special Characteristics
Play-of-color; thin opal layers naturally bonded to host rock (Matrix Opal); occasionally fluorescent
Lore & History
Discovered in Queensland in the late 19th century; culturally associated with communication and emotional clarity