You could make it out of tungsten, but it'd be heavy, brittle, and an absolute bastard to machine. It'd probably increase the breakeven cost by an order of magn

You could make it out of tungsten, but it'd be heavy, brittle, and an absolute bastard to machine. It'd probably increase the breakeven cost by an order of magnitude. For protecting the surface finish of the gold, you'd probably want something like the polycarbonate cases you sometimes see used for rare or precious coins.