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Iolite is the deep blue gem variety of the silicate mineral cordierite, named for French geologist Pierre Cordier. Iolite is found on Garnet Island in Canada and in South Asia and South Africa. Leif Eriksson and other Viking explorers are said to have used thin slices of iolite to navigate during voyages, taking advantage of its prismatic properties to polarize sunlight. Opals form when silica gel hardens in the cracks of other rocks. Most of the world's precious opal comes from Australia, where opal is the national gem. A boulder opal is an opal that has been sliced together with its surrounding ironstone for a varied look. Fire opal from Mexico comes in shades of orange and Peruvian opal is an opaque blue-green stone. An opal doublet has polished precious opal on the top with an ironstone backing added for strength, making the slice of opal less prone to chipping. Derived from "tapaz," the Sanskrit word for fire, topaz is one of the most brilliant cut gemstones in existence. Topaz comes in a variety of colors from yellow to blue, and deeper blues and pinks are often created by heat-treating stones. Brazil is the world's top exporter of topaz. Beautiful turquoise is a phosphate mineral popular all over the world. Mined since 5000 BC, it is one of the first gemstones known to be used by humans. Color variation from sky-blue to medium green is affected by differing amounts of copper and iron, more copper giving the stone a bluer color and more iron a greener one. Beautiful light blue turquoise is mined in Northern Iran and other varieties occur in Mexico, the United States and elsewhere.
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