Opal
Image: James St. John (CC BY 2.0)
Mineral NameOpal
Chemical FormulaSiO₂·nH₂O
Mohs Hardness6
Crystal Systemamorphous
Lustervitreous
Streak Colorwhite
Cleavagenone
Fractureconchoidal
Specific Gravity2.1
Colorswhite, colorless, yellow, orange, red, green, blue, pink, brown, black
Transparencytranslucent
Type LocalityWhite Cliffs Opal Field, New South Wales, Australia
Mineral GroupSilicates
Usesjewelry, collector
Associated Mineralsquartz
Rarityuncommon
Description
This image displays a piece of precious opal embedded within a light-colored sandstone matrix. The opal exhibits a vibrant play-of-color, showcasing flashes of green, blue, red, and orange against a translucent, whitish body.
Geological Context
Formed in the Wallumbilla Formation during the Lower Cretaceous period, this precious opal occurs within quartzose sandstone, typically filling voids or fractures in sedimentary rocks. It forms from the precipitation of silica-rich solutions.
