It might seem illogical that the development of modern currency rests on a scientific discovery, but the invention of the "touchstone" allowed ancient societies to create a standard by which valuable metals could be judged. In its most basic form, a touchstone is any dark, finely grained stone upon which soft metals leave traces. When rubbed, a process known as "probing," precious metal alloy cleaves to the stone, leaving a stripe. The color of the stripe (which reveals the percentage of its content that is base metal) can then be compared to a stripe of a known grade of standard alloy. Despite its primitiveness, this probing process allowed merchants to examine alloys quickly and with reasonable certainty. Though civilizations were using gold and silver currencies as early as 500 b.c., coins were easily forged or diluted with less valuable metals, such as tin or lead. The invention and popularization of the touchstone ensured that pure gold and silver could become a standard expression of value.