What: Shares of miner Freeport-McMoRan Inc (NYSE:FCX) plunged as much as 10% today after copper prices hit a low on the market.

So what: Copper has fallen off a cliff in July, dropping 7.1% for the month, and it was down 4.7% at midday today according to Bloomberg. Commodities in general are getting pounded because China's markets are falling off a cliff, and there's worry that the economy there could slow down significantly. If it does, commodities like copper and oil will be among the worst hit, and that's bad news for Freeport-McMoRan's operations in an already difficult operating environment.

Now what: After losing more than $5 billion in the last two quarters, the last thing Freeport-McMoRan needs is plunging commodity prices. But that's what the market is giving it, and with China's future in flux, it's hard to know if/when there will be a recovery.

This is the risk in owning commodity stocks, so it shouldn't surprise investors, but it'll be important for Freeport-McMoRan to show it can adjust its cost structure in coming quarters if prices remain low. No reason to jump ship today, but watch management's comments and operating results after the next quarterly earnings release for signs the company is adapting well or sinking under commodity pressures.