Touchstone and gold stars.
© Science Museum / Science & Society Picture Library
Touchstones were used for many years to judge the purity of gold. A goldsmith would compare the streaks made by a potential piece of gold on the touchstone to those made by real gold. The points of the stars were all marked by gold of different purities, which allowed for greater refinement. The touchstone was usually a fine-grained dark schist or jasper.