Options traders who bet the Trump administration would renege on its promise to tariff raw copper imports are cashing in big.

Copper traders were caught flat-footed after refined copper was exempted from Trump's new tariffs. The exemption triggered a 20% drop in copper futures on the COMEX exchange. Traders now face excess inventory, with COMEX stockpiles at a 21-year high.

The president stopped short of putting a 50 percent tariff on refined copper, as he had threatened earlier this year. The market reaction was swift anyway and prices tumbled.