Australian Doublet Opal Pendant Set in Sterling Silver. Genuine Natural Australian Gem Doublet Opal is a hydrated amorphous form of silica SiO2�nH2O consisting of spheres of silicon dioxide molecules arranged in regular, closely packed planes backed by Boulder Opal ironstone (found in remote areas in Queensland Australia). The two layers are bonded with a silica-based adherent which has the same coefficient of expansion as the layers allowing consistant expansion and contraction which keeps the Opal layers free from delamination when coming in contact with water. Guaranteed free of dyes and artificial colour. Doublet Opal is categorised as Natural Opal Type 3.