Formation of Opal
Light of the World, Pandora, Queen of Gems – or more commonly, opal is a strongly hydrated form of silica characterised by its amorphous structure. This luminous gem occurs in the fissures and layers of underground sedimentary rocks and is a by-product of floods and heavy rains that cause water to filter through the ground, leaving deposits of silica behind. Although opal can form in the cracks of nearly every rock, it is most commonly found in basalt, sandstone, marl, rhyolite and limonite. Depending on the location and circumstances in which opal formed, the stone can be either transparent, translucent or opaque, with white, black or polychromatic background. Majority of the gems are either white, grey or green, while the black opal is considered to be the rarest.