GERLACH, Nev.—Like generations of Nevada prospectors before her, Shelly Egbert plunged her shovel into her mining claim one recent autumn afternoon. She hit a mother lode.

"Look at that," she yelled, nodding at the bounty on the flat of her spade. "Straight from the center of the earth!"

It was earth, as a matter of fact. "Smooth as butter,"...