Touchstone

The metaphorical or idiomatic usage of touchstone is to describe anything which serves to test the genuineness or value of anything; a test, a trial; a criterion or reference point by which something is assessed, judged, or recognized. For example, " Tolstoy's War and Peace is the touchstone of historical fiction." This usage dates from 1533, according to the Oxford English Dictionary. The origin of touchstone, however, dates back to antiquity. A touchstone is a small tablet of dark stone such as slate or lyddite, used for assaying precious metal alloys, especially gold. A touchstone has a finely grained surface on which soft metals like gold leave a visible trace. Thus, in ancient times, from as early as 2000 BC, and in ancient Greece, touchstones were used to test if something was real gold. In other words, it was an ancient form of the acid test. using aqua fortis, or nitric acid, as it is now known. See also acid test and litmus test.