Opal is found all over the world (Brazil, Mexico, Honduras and the western United States), however, Australia produces 95% of the world's precious opal and is our official national gemstone. Queensland's Boulder opal fields cover an area of many thousands of square kilometers (Ratliff, 201). Sunflower opal is a term that is sometimes erroneously and incorrectly used to refer to fire opals, as well as a transparent to semi-transparent type of Madagascar milky quartz that shows an asterism or star effect when properly cut. Australia's rugged and desolate interior provided unique conditions for silica-rich water to seep deep into the Earth's crevices, initiating the epically long process of opal formation.
In an attempt to match the brightness of Australian precious opals, Ethiopian opals are often chemically treated to increase the intensity of their color. In addition to thunder eggs, Oregon supplies a variety of fire opals, as well as common opals, rainbow opals, centerlight opals, blue opals, and dendritic opals. Although there are many opal mining towns in Australia, there are four that have become household names: Coober Pedy, Andamooka, White Cliffs and Lightning Ridge. Due to Australia's vast geographical and landscape diversity, it is not enough to classify all of Australia's opals as “Australian opals”.
Higher temperature polymorphs of silica cristobalite and tridymite are often the first to crystallize from amorphous anhydrous silica, and the local structures of microcrystalline opals also appear to be closer to those of cristobalite and tridymite than to quartz. In contrast, Ethiopian opals are porous in nature, making their inherent structure less durable than a natural, untreated Australian opal. The world's largest and most valuable opal Olympic Australis was found in August 1956 on the Eight Mile opal field in Coober Pedy. There are three types of opal mined in Australia and they are white opal, boulder opal and black opal.
White opal is a precious opal found in common white opal and characteristically occurs in lighter colors than black opal as a result of being housed by common white opal. These opals are found in impressive geodes, rocks that seem simple on the outside but that explode with color and diversity inside. White Cliffs is also notable for producing “opal pinecones”, a strangely shaped opal fossil in the shape of a mineral crystal.
