A slab of white,iridescent opal. Opal is composed of microspheroids of hydrous silicon oxide. It is the refraction of light within these spheroids which gives the mineral its iridescent,mother-of- pearl-like sheen. Opal never occurs in a crystaline form,but as small veins,globules and crusts. Several varieties occur; colourless,milky white,hazy blue,red (fire opal) and black types and also a type of fossilised wood know as wood opal. Black and fire opals are particularly valued as gemstones. This specimen measures 9 x 7 cm and was found in Pedro II,Piaui,Brazil |