Composition: SiO2 nH2O hydrated silicon dioxide. Water content: 3-10% (up to 20%).
Look:
Colour: virtually of each colour.
The base colour results from a trace of Ni, Cr, Fe, Mn and the inclusion of other minerals.
The colour effect results from the dimensions of the sub microscopic spheres.
Transparency: from transparent to opaque.
Shine: glazed to resinous.
They are presented in kidney and grape (botryoidal) -shaped aggregates, in venules, globules, crusts and blotches of colourless filling, milk-white, often bluish or black with variable opalescence; compact and earthy masses, often concretional and stalactitic (diatomite, geyserite).
Phenomenon: colour effect (opalescence), asterism (rare), glittering (rare).
Crystalline state: amorphous.
Hardness: 5,5 – 6,5.
Tenacity: from very scarce to sufficient.
Density: 1,9 – 2,3; porous 1,26.
Specific weight: 2,15 (+0,08; - 0,90).
Fracturing: from conchoidal to non-uniform; splintery.
Lamination: none.
Optical character: mono-refracting, with strong anomalous double refraction due to internal tensions.
Refraction index: 1,44 - 1,46; porous 1,37. The Mexican opal can give a 1,37 reading. Usually 1,42-1,43.
Fluorescenc:
White opal: from inert to white to moderate blue, green, or yellow.
Black opal: generally inert.
Fire opal: from inert to moderate greenish brown
Common opal: from inert to strong green or yellowish green.
All opals can show some luminescence.
Colour of rubbing: white, yellowy, browny.
Dispersion: none to very low.
Shininess of a fracture: from subvitreous to cerulean (the second is typical of a common opal).
Absorbency range: green-660 nm, 470 nm demarcation line.
Typical cut styles: cabochon, engraving, faceted, sphere. The noble opal is rarely faceted due to the substantial loss of material during the cutting procedure. The fire opal, however, often undergoes this procedure as it accents the high transparency. |
