Opal
Image: JJ Harrison (https://www.jjharrison.com.au/) (CC BY-SA 2.5)
Mineral NameOpal
Chemical FormulaSiO2·nH2O
Mohs Hardness5.5
Crystal Systemamorphous
Lustervitreous
Streak Colorwhite
Cleavagenone
Fractureconchoidal
Specific Gravity2.1
Colorsmulticolored, brown, black, gold
Transparencytranslucent
Type LocalityYowah, Queensland, Australia
Mineral GroupSilicates
Usesjewelry, collector
Associated Mineralsironstone, sandstone
Rarityuncommon
Description
This polished boulder opal cabochon displays vibrant flashes of blue, green, and purple play-of-color embedded within a dark brown to golden-brown ironstone matrix. The precious opal forms intricate patterns and veins throughout the host rock.
Geological Context
Opal forms from the precipitation of silica-rich solutions in cracks and voids within host rocks, often sedimentary rocks like sandstone or ironstone. Boulder opal specifically occurs as veins or patches within ironstone concretions, common in the arid regions of Queensland, Australia.
