Barrier reef

Chemical Composition

The composition of opal is variable. Chemically, it is defined as a hydrated form of silica (SiO2.nH2O), with water content between 1% and 21%. Precious opal usually contains 6% to 10% water.


Opal colour ranges across the visible spectrum, from deep red to violet. Precious opal may be transparent, translucent or opaque with patches or speckles of bright spectral colours which change or disappear as the stone is rotated.

Crystal Form

Opal does not crystallise in a regular form, and is amorphous. It may be considered as solidified silica gel. Most New South Wales opal is hosted by sedimentary rock and is without crystal form.

Other Properties

Hardness: 5.5–7
Specific gravity: 1.98–2.20; 2.1–2.2 (precious opal)
Lustre: Vitreous to sub-vitreous. Common opal is frequently resinous.
Refractive index: 1.44–1.46
Streak: White
Fracture: Conchoidal
Diaphaneity: Transparent to opaque
Cleavage: Absent

For further information
Lightning Ridge Office
+61 (0)2 6820 5200
+61 (0)2 6829 0825
NSW Department of Planning, Industry and Environment, Division of Resources and Geoscience, PO Box 314 Lightning Ridge NSW 2834
Lot 60 Morilla Street Lightning Ridge NSW 2834 Map