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.

Well one historic way of doing this, was instead of trading directly in minute quantities of gold, people would store larger amounts of gold in a bank and trade contracts that represent certain fractions of what is stored. This was called "currency".

Interesting point. I don't have any comment, but it looks like this guy disagrees with you: https://news.ycombinator.com/item?id=31933490