Please note that the photos were taken on both dry and wet opal. Obviously, the wet opal is brighter and more colorful, highlighting the effect that can be achieved after polishing. In Australian mining practice, rough opal is conventionally photographed wet, not to mislead buyers, but to represent the potential of the stone. For this reason, we encourage you to carefully evaluate the photographs before bidding. Thank you for your attention and happy bidding! Natural Australian Rough Opal Dimensions from 9 to 21 mm approx 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.