- 1: a black siliceous stone related to flint and formerly used to test the purity of gold and silver by the streak left on the stone when rubbed by the metal
- 2: a test or criterion for determining the quality or genuineness of a thing
- 3: a fundamental or quintessential part or feature : basis <a touchstone film of that decade> <now considered a touchstone of the city's life — Michael Specter>
The touchstone was used in ancient Greece. Its role in the introduction of monetary economy was explored by science historian James Burke in the second episode of his 1978 BBC television series Connections.
It was also used by Indus Valley Civilization about 3500 BC for testing the purity of soft metals.
Drawing a line with gold on a touchstone will leave a visible trace. Because different alloys of gold have different colours (see gold) the unknown sample can be compared to samples of known purity. This method has been used since ancient times. In modern times, additional tests can be done. The trace will react differently to specific concentrations of nitric acid or aqua regia, thereby identifying the quality of the gold. Thus, 24 carat gold is not affected but 14 carat gold will show chemical activity.
As a metaphor, a touchstone refers to any physical or intellectual measure by which the validity or merit of a concept can be tested. It is similar in use to an acid test, litmus test in politics, or, from a negative perspective, a shibboleth where the criterion is considered by some to be out-of-date. The word was introduced into literary criticism by Matthew Arnold in "the Study of Poetry" (1880) to denote short but distinctive passages, selected from the writings of the greatest poets, which he used to determine the relative value of passages or poems which are compared to them. Arnold proposed this method of evaluation as a corrective for what he called the "fallacious" estimates of poems according to their "historic" importance in the development of literature, or else according to their "personal" appeal to an individual critic.