Drawing a line with gold on a touchstone will leave a visible trace. Because different alloys of gold have different colours 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.
The term is derived from the Middle English bellewether and refers to the practice of placing a bell around the neck of a castrated ram (a wether) leading his flock of sheep. The movements of the flock could be noted by hearing the bell before the flock was in sight.
Quark, for example, was formerly a nonce word in English, appearing only in James Joyce’s Finnegans Wake. Murray Gell-Mann then adopted it to name a new class of subatomic particle.