amorphous Mineraloid
Opal
SiO2.nH2O
Quick Facts
Hardness
5.5
Specific Gravity
1.98
Refractive Index
1.37
Optical Character
Isotropic
Crystal Structure
#! Species: Opal
#! System: Amorphous (mineraloid)
#! Habit: Botryoidal masses with play of color
amorphous[opalescent]:{botryoidal} | phenomenon[play_of_color:intense]
amorphous
none
{botryoidal}
Physical Properties
Crystal Systemamorphous
Hardness (Mohs)5.5
Specific Gravity1.98
CleavageNone
FractureConchoidal
LustreVitreous to resinous
Optical Properties
Refractive Index1.37
Optical CharacterIsotropic
PleochroismNone (amorphous)
PhenomenonPlay of color (precious opal)
Colours
White body (white opal)Black body (black opal - most valuable)Transparent (crystal opal)Orange-red body (fire opal)Boulder matrixPlay of color (spectral flashes)
Localities
Australia (Lightning Ridge - black; Coober Pedy - white)Ethiopia (Welo - hydrophane)Mexico (fire opal)BrazilUSA (Nevada, Oregon)Honduras
Known Treatments
Smoke treatmentSugar/acid treatmentPolymer impregnationDoublets and triplets
Crystal Forms
botryoidalnodulesveinsreplacement forms
Diagnostic Features
Play of colour (precious); amorphous; low hardness (5.5-6.5); hydrated silica
Notes
Mineraloid — no true crystal structure; play of color from diffraction in ordered silica sphere arrays
Interactive Preview
Open this crystal in the CDL Playground to rotate, export, and explore in 3D.
Open in Playground #! Species: Opal
#! System: Amorphous (mineraloid)
#! Habit: Botryoidal masses with play of color
amorphous[opalescent]:{botryoidal} | phenomenon[play_of_color:intense]
Synthetics & Simulants
Known synthetic and simulant versions of Opal
Related Minerals
Other minerals in the amorphous system
