A touchstone is a small rock of a material similar to fieldstone or slate. It has a rough surface that can be used for grinding.
Use of the touchstone revolutionized the concept of money. Prior to its introduction gold and silver were common currencies, but these could easily be alloyed with a less expensive metal (tin and lead were common). These were less valuable, but it was difficult to test for.
But by rubbing the metal on a touchstone a small amount would be ground off onto the stone, and form a colored stripe. The color varied depending on the amount of the base metal, making it easy to test for alloys. Typically the user first primed[?] the stone by having a stripe from a known high-quality source rubbed into the stone at the top, typically for a small fee. One could then test the quality of any metal by comparing the two colors.
Today, thousands of years later, the term is used to describe a common gemstone.
