Australian opals (SiO₂ nH₂O) are among the most precious in the world, renowned for their play of color ranging from electric blue to emerald green and fiery red. They are formed through the deposition of amorphous silica in ancient sedimentary rocks. The main mines are located in Coober Pedy, Lightning Ridge (famous for its black opals), and Queensland, renowned for Boulder and Koroit opals, ending in South Australia, home to the beautiful Andamooka opals.