Opal
Image: Robert M. Lavinsky (CC BY-SA 3.0)
Mineral NameOpal
Chemical FormulaSiO₂·nH₂O
Mohs Hardness5.5
Crystal Systemamorphous
Lustervitreous
Streak Colorwhite
Cleavagenone
Fractureconchoidal
Specific Gravity2.1
Colorscolorless, pale yellow, white, iridescent
Transparencytransparent
Type LocalityVirgin Valley District, Humboldt County, Nevada, USA
Mineral GroupSilicates
Usesjewelry, collector's item
Associated Mineralsvolcanic ash, chalcedony, quartz
Rarityuncommon
Description
A large chunk of pale yellowish-white opal, featuring a prominent transparent 'jelly opal' section with internal fractures displaying iridescence and a glowing play-of-color.
Geological Context
Opal forms from the precipitation of silica-rich solutions, often in volcanic ash beds or sedimentary rocks, where silica gel hardens over time. Virgin Valley opal is typically found in altered volcanic ash.
