Density Testing For Gold And Silver...

Hi all

I have recently been buying and selling gold and silver and haven't yet had a fool proof way of testing its real.

Out of curiosity, am I right in saying, each metal has different density, so the exact same size block of metal, would always way different to other metals?

If so, why has there not been a method of testing invented to divide the volume by weight? am I missing something to stop it being this straight forward?

Obviously would need to create something to get exact volume, which would be easy with new technology, putting gold in some kind of liquid and digitally get the volume the liquid has risen, and divide that by the weight to find out if correct density?

Would this not do the trick? Or is my understanding a little off somewhere?

Thanks