Silver Bar

Silver is roughly where the early game stops fighting back. The pickaxe digs fast enough that tunneling is no longer a chore, and the armor holds up on a first serious trip underground instead of folding to the first skeleton.

The bars keep earning after that. Silver Bullets are the obvious use if you are running a gun at all, and turning a few spare bars into ammo beats leaving loose ore in storage forever. The gem staff recipe is the other one worth remembering, since a Sapphire Staff is a real weapon that early in a run and it is easy to walk past.