Black, silica-containing stone used in assaying to determine the purity of gold and silver. The metal to be assayed is rubbed on the touchstone, and then a sample of metal of known purity is rubbed on the stone right next to it. The streaks of metal on the stone are treated with nitric acid, which dissolves impurities, thus increasing the contrast between the two samples when compared. Because other metals, such as copper, can be alloyed to silver without changing its colour significantly, the touchstone method is not usually used now to assay silver, though it is still used to assay gold and provides a reasonably accurate guide to quality.
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