Which single element are they then?

Surely such a system would have a products table with something like nominal element fk, purity, mass columns anyway. (And stock table with fk to products.)

Then nothing wrong with 'natural' key on the elements, but sure, products are the thing you've (personally) made up, so of course there's no 'someone else's' key to use.

Diamonds and coal are both pure carbon (mostly carbon-12). So is graphite, and buckminsterfullerene, and a number of other things. Crystal structure matters a lot.

They're not pure carbon was my point. I wasn't saying crystal structure not important, I missed that but it'd be in my 'product derived from element x' table.