Australian Natural Boulder Opal pendant set in sterling silver. Boulder Opal is Classified by the Opal industry as Natural Opal Type 2 Boulder Opal is the definition applied where the Opal is naturally attached to the host rock in which it was formed and the host rock is of a different chemical composition. Genuine Natural Australian Boulder Opal consists of a hydrated amorphous form of concretions of SiO2nH2O silica consisting of spheres of silicon dioxide molecules arranged in regular, closely packed planes filling the cracks and crevices of Boulder Opal ironstone found remotely in Queensland Australia. Boulder Opal is a diminishing resource with anecdotal evidence suggesting increases in value of up to 15% per annum. Guaranteed free of dyes and artificial colour.