Same volume, different weight
When the geometry stays the same, the volume doesn’t change. What changes is the material that fills it. So a weight calculation from CAD volume has to refer to the relevant metal or alloy.
Why not settle for the number?
A mathematical calculation doesn’t know whether the result is reasonable for a piece of jewelry. Run a sanity check against the type of product, its dimensions and its structure - and review the model if the number looks out of line.
The choice of metal can affect production, too
In my experience in Israel, 18K white gold can behave differently in casting from 14K white and sometimes needs different handling at the production-file level. It is one example of how the material is more than a figure in a weight calculation.
Platinum highlights the importance of the production chain
Platinum requires a casting house and a craftsperson experienced in working with it. A model can be right, but if the production process doesn’t suit the material, the result may still not be the one intended.