Excellent Natural Ethiopian Rough Opal. White base with multicolour fire. Top stones for collectors. Dimensions from 17 mm to 26 mm. All pictures are taken with high resolution camera on bone dry material and on different backgrounds in order to depict the opal as realistically as possible. Worldwide shipping with registered mail and tracking number. An opal is a hydrated form of silica with a water content ranging from 3 to 21%. Indeed, it is this permeability to water that makes it a hydrophile. Opals are deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt. Only precious opals show a variable interplay of internal colors. At microscopic scales, precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close-packed lattice. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal.