However I have a problem here: The Manufacturer may know what gemstone they are going to place in any given setting, but as the diamond size may be different from say a Sapphire (weight for weight) how can you be sure this is correct?

A diamond may have been swapped for a Cubic Zirconia of the same size - but the weights will be quite different.

So how can you be sure it is what you think it is?

We are required in the US to stamp this by law, although not everyone does it.

My firm has been doing it for decades, so it should be quite common by now.

Hope that helps