Australian Opal Cutters Doublet Opal Pendant 925 Sterling Silver Classified by the Opal industry as Natural Opal Type 4 - Doublet Opal is Genuine Natural Australian Gem Crystal Opal, a hydrated amorphous form of silica SiO2nH2O consisting of spheres of silicon dioxide molecules arranged in regular, closely packed planes. Backed by Australian Black Opal potch or Boulder Opal Potch. The two layers have the same coefficient of thermal expansion and are guaranteed for life against delamination. Guaranteed free of dyes and artificial colour.