Image: James St. John (CC BY 2.0)
amorphousuncommonSilicates
Opal
SiO₂·nH₂O
This specimen is a precious opal exhibiting a distinctive 'opal pineapple' pseudomorph structure, replacing glendonite, with a white body color and iridescent play-of-color.
Opal Physical Properties
Mohs Hardness
6Crystal System
amorphousLuster
vitreousStreak Color
whiteCleavage
noneFracture
conchoidalSpecific Gravity
2.1Colors
white, yellow, orange, red, green, blue, black, colorlessTransparency
translucentType Locality
White Cliffs Opal Field, New South Wales, AustraliaMineral Group
SilicatesUses
jewelry, collectorAssociated Minerals
glendonite, chalcedony, quartz, calciteRarity
uncommonGeological Context
Opal forms from the precipitation of silica-rich solutions at low temperatures, often in sedimentary rocks. This specific specimen is a pseudomorph, where opal has replaced pre-existing glendonite crystals, which themselves are pseudomorphs of calcite after ikaite.
