Going back in history 4500 years, give or take a few – some genius merchant or trader figured out a simple and accurate way to test the purity of gold.
The “touchstone” was a piece of dark schist type rock - which was used for testing the putative gold by observing the color of the mark left on the stone when lightly rubbed (touched). Gold can be alloyed with copper and other cheaper base metals without changing its color very much – but when that alloy is in tested on a touchstone, it becomes immediately apparent that the color has changed - if and when the gold has been diluted with any small amount of another metal. Supposedly the technique is almost foolproof for an experienced merchant – and it is still used today by jewelers and coin collectors. This may have some slight relevance to the Mizuno technique since both involve noble metals and a surface effect of mechanical contact. It does not take much in the way of contact of loss to get an answer. A tiny amount of a precious metal will apparently make itself known in ways which are not obvious. OK – it’s a slow News day…
