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Zircon starts off as molten magma, the hot lava from volcanoes. This magma then crystallizes. Andwhen Zircon crystallizes, the Uranium inside it begins to change into Lead. Soif you measure the amount of Lead in the Zircon grain, you can figure out whenthe grain was formed. After that, you can determine the age of Zircon fromdifferent mountain ranges. Once you do that, you can compare the age of theZircon in the sandstone in your sample to the age of the Zircon in themountains. If the age of the Zircon matches the age of one of the mountainranges, then it means the sandstone actually used to be part of that particularmountain range.