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OPAL

           
 
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OPAL
Gemstone:
Shape:
Weight (ct):
 to
 
 
Opal Facts 
Category Mineraloid
Chemical formula Hydrated silica. SiO2·nH2O
Identification
Color White, black, red, orange, most of the full spectrum, colorless, iridescent. Very infrequently of a singular color
Crystal habit Irregular veins, in masses, in nodules
Crystal system Amorphous[1]
Cleavage None[1]
Fracture Conchoidal to uneven[1]
Mohs Scale hardness 5.5–6.5[1]
Luster Subvitreous to waxy[1]
Polish luster Vitreous to resinous[1]
Refractive index 1.450 (+.020, -.080) Mexican Opal may read as low as 1.37, but typically reads 1.42–1.43[1]
Optical Properties Single refractive, often anomalous double refractive due to strain[1]
Birefringence none[1]
Pleochroism None[1]
Ultraviolet fluorescence black or white body color: inert to white to moderate light blue, green, or yellow in long and short wave. May also phosphoresce; common opal: inert to strong green or yellowish green in long and short wave, may phosphoresce; fire opal: inert to moderate greenish brown in long and short wave, may phosphoresce.[1]
Absorption spectra green stones: 660nm, 470nm cutoff[1]
Streak White
Specific gravity 2.15 (+.08, -.90)[1]
Diagnostic Features darkening upon heating
Solubility hot saltwater, bases, methanol, humic acid, hydrofluoric acid
Diaphaneity opaque, translucent, transparent
References [2][3]
Diaphaneity Transparent to opaque
References [1][2]