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Amber, an organic gemstone, is the fossilized resin of prehistoric trees which grew up to 50 million years ago. The stone has been used for jewelry since the time of Christ. The world's finest amber comes from the region around the Baltic sea. 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. 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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