To my knowlege, there is no such thing, legally, as a precious metal.
Nor is there any such thing, legally, as a precious gem. It is social
convention, and yes, as you say, the fact that it must be stamped is
a line in the sand. Nor is there any such thing, legally, as an
industrial metal, to my knowlege. What there is, legally, is a list
of "Strategic Metals", which are necessary for the national defense,
and are restricted in time of need. Platinum, rhodium, gold, silver,
copper are all strategic metals, as are molybdenum, titanium, cobalt,
I could just be ignorant about this, but as far as I know, there is
no legal definition of a precious metal per se. There was a big stink
years ago about how Tiffany's wouldn't call turquoise "precious", and
that was impacting sales and stuff. Now, if Tiffany's can be
considered a legal authority on the matter, maybe we should all find
another line of work. Influential, yes, legal, no.