Opal is an amorphous mineral with a water content of 5-10%. It is formed when colloidal silica gel is deposited slowly at a low temperature.
It never appears in crystal form but in veins, nodules and crusts with wonderfully iridescent colours, produced by the diffraction of light on silica spheres.
It owes its distinguishing elusive, mysterious, magical appearance to its opalescence.
Australia produces 97% of the world’s opals, hence the opal is its national stone.
Other deposits are to be found in Mexico, the area of the Czech Republic and Slovakia, and Brazil.