Hi Shayne,

Here's a FREE Mass Calculator Plug-in for Rhino that works quite well.

http://www.rhino3d.e-cnc.com/masscalc/masscalc.htm

It has a good selection of the various Precious Metal Alloys we use & even some we don't. ;)
Just be aware that your model or items for calculation need to be a solid.

HTH, Colin