Natural Ethiopian Cab Opals Clarity: from translucent to opaque; Pictures taken with high resolution camera on different backgrounds in order to depict the opals 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. 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.