MP Materials, the operator of the only rare earths mine in the United States, has signed a $500 million deal with Apple.

Apple has signed a $500-million deal with Pentagon-backed MP Materials for a supply of rare earth magnets, becoming one of the first tech companies to ink a U.S. supply agreement after China curbed exports earlier this year.

MP Materials’ goal to become a national rare-earths “champion” that can go toe-to-toe with China was turbocharged by Pentagon investment and an Apple deal. But its journey there highlights the hurdles American producers face.