For daily transactions at the current gold price, it would be extremely hard to do this sort of measurement that way with any precision without very specialized equipment. A gram of gold (around $60) is a reasonable daily transaction size and has a volume of 0.05ml. In order to distinguish metals of similar densities, you'd have to be able to measure just a fraction of that volume accurately. Maybe there is some clever system to scale this down, but wasn't obvious to me.