Touchstone

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.