An amorphous silica mineraloid with the chemical formula SiO2 .nH20. Opal comes in many unique varieties based on the location it was sourced from. There are two basic types of opal that is described by the visual appearance, precious opal and common opal. Precious opal exhibits the phenomenon known as play-of-colour. Common opal and potch, are opals that do not exhibit this phenomenon. The majority of opal is mined in Australia where over 96% of the worlds supply is produced. Notable mines in Australia include Coober Pedy, Lightning Ridge, Lambina, Mintabie, Andamooka, Yowah, Koroit, Jundah, Winton and Quilpie. Other countries that produce both precious and common opal include; Brazil, Czech Republic, Ethiopia, Guatemala, Honduras, Hungary, Indonesia, Mexico, Nicaragua, Peru, Slovakia and the U.S.A.