Opal, close-up. Opal is an amorphous hydrated silicon dioxide (Si02.nH20). It is usually deposited in fissures and voids inside a variety of host rocks, most of volcanic origin. Opal can have beautiful soft colouring and is highly sought after as a precious stone in jewellery. The piece shown here is the polished surface of a thin slab that was artificially composed by immersing small morsels of opal in a resinous binder. Magnification: x1.8 when printed 10 centimetres wide.