Historically, the term touchstone referred to a physical stone that was used for assaying precious metals, for determining the purity of an alloy. The touchstone was a small tablet of dark stone such as fieldstone, or slate. The stone had a finely grained surface, so that a soft metal rubbed against it would leave a visible trace. A pure piece of silver would produce a recognizable color. Another alloy rubbed alongside the silver mark would leave a different colored trace. A merchant could compare the two colors to determine the purity of a piece of metal being offered for trade.
