Opal Information

What is Opal?

Australian opal which makes up 95% of the world's opal, is an amorphous, non-crystalline silica, (silicon dioxide, similar to quartz and sand) a hydrated form of silica (SiO2H2O) in a rigid gel form containing between 1% and 21% water. Precious opal usually contains 6% to 10% water. The water is located in tiny voids between the spheres that are so tiny that you need an electron microscope to see them and they are held so tightly that it is nearly impossible for them to escape.

Silicate minerals in the stone add to its weight giving it a specific gravity of between 1.98 and 2.2 of pure water (precious opal is between 2.1-2.2).

Opal’s scratch hardness on the Moh scale ranges from 6.0 to 6.5 which is between the hardness of a moonstone and quartz and its refractive index varies from 1.44-1.46

Opal is a closely packed array of billions of spherical particles stacked in a three dimensional grating that has the unique ability to diffract white light into beautiful colours of the rainbow without any impurities. The colour is created when light is split by the voids between the spheres.

The diameters of the spheres therefore determine the maximum size of the wavelength or colour that can be developed. Visible colour spheres must be no smaller than 1500 angstroms for violet, indigo and blue and no larger than 3500 angstroms for orange or red.

Therefore a stone that can display red can display all shorter visible wave lengths, orange yellow green and blue.

Opal in Australia was formed about 60 million years ago when the deserts of central Australia were a great inland sea with silica rich deposits situated around its shoreline. Over time climatic change caused the sea to recede and disappear becoming the great artesian basin. During this time a solution subsequently was deposited in open cavities and cracks in sedimentary rock, around the Cretaceous period or the Age of the Dinosaurs, (explaining the occasional discoveries of prehistoric opalised skeletons, wood and shells).