Touchstones were stones used to test the purity of precious metals such as gold and silver. Touchstones were also called Lydian stones, after the country of Lydia, the birthplace of precious metal coinage and the first country credited with the use of touchstones. The spread of gold coinage particularly increased the profits that could be earned from adulterating and alloying gold coinage, and touchstones offered an inexpensive and useful test for purity of gold coinage. Both individuals and governments were known to reduce the purity of precious metals by alloying them with cheaper metals.
Touchstones were cut from black siliceous stone or opaque quartz, brown, red, or yellow in color, with a smooth surface, and convenient for holding in one hand. Ancient and medieval assayers tested the purity of gold or silver by rubbing the metal across a touchstone with sufficient pressure to leave a streak. Different metals left streaks of different colors. The color of the streak left on the touchstone by a metal of unknown purity could be compared with the color of a streak left by a piece of metal of known purity. Nitric acid was put on the streaks to dissolve impurities, and sharpen the contrast between the streaks of pure and impure metal. From this comparison an assayer rendered a judgment about the purity of a metal. Because differences in shades of color can be slight, the test involved a significant subjective component. Nevertheless, the test brought to light the more outrageous debasements, and was sufficiently accurate for most purposes.
Before the development of more advanced techniques the Goldsmiths’ Company of the City of London kept test metals of known purity, called touch needles, for use in making touchstone tests. The company made available 24 gold needles for each of the traditional 24 gold carats. They kept similar pieces for silver.

Touchstone tests are not decisive in detecting silver alloyed with copper, but can be used to assay gold with some accuracy. By the fifteenth century the Tower mint in London was using a new method, cupellation, which makes use of the tendency of various metals to fuse at high temperatures. The new method using fire grew out of the experiments of the alchemists during the medieval era.

See also:
Trial of the Pyx (England)
Davies, Glyn. 1994. A History of Money.
Marx, Jennifer. 1978. The Magic of Gold.