3 ideas
Every substance has a fixed ratio of weight to volume, so a sample of a stone or metal can be checked by comparing its weight against an equal volume of water. Measurement therefore replaces the dealer's eye and the seller's word as the test of what a thing really is.
The value of gems and precious metals rests on scarcity, fashion and agreement among buyers, not on any usefulness in the thing itself. Prices recorded from merchants move with place, era and the whims of rulers. So a gem's price describes the society trading it more than the stone.
Putting physical fact beside market testimony and known deceptions gives a buyer both the truth about the object and a map of how it is misrepresented. Merchants' reports are treated as evidence to be checked against measurement, not as authority.