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.
added 2013-08-06T16:46:21Z by anders