Falling copper and oil hit Freeport today. Image: Freeport-McMoRan.

After a terrible week to start 2016, investors came into Monday's session hoping for a bounce-back. Early gains quickly faded in the early afternoon, but by the end of the day, the Dow and S&P 500 had both worked their way back into positive territory. Still, plunging oil prices hit levels not seen in more than a decade, and a good number of stocks kept falling. Among the worst performers were Freeport-McMoRan (NYSE:FCX), Fitbit (NYSE:FIT), and A. Schulman (NASDAQ:SHLM).

Freeport-McMoRan lost more than 20% as the natural resources company faced continuing weakness in both of its key markets. Oil's plunge to around $31 per barrel continued to weigh on Freeport's energy business, which will likely face at least one more round of asset impairments based on the much-higher carrying value of its reserves. Moreover, copper prices fell below $2 per pound to hit their lowest levels since 2009, dealing a blow to Freeport's mining business. With concerns in China continuing to convince investors that sluggish demand in the commodity sector will persist well into the year and potentially beyond, few investors see Freeport being able to recover anytime soon, and many of its peers in the mining sector also posted substantial declines.

Fitbit dropped 12%, extending its recent losses and falling below its initial public offering price. The wearable-technology company has been unable to convince investors that its Blaze smartwatch will be able to keep its competitors at bay, raising concerns that the company's margins will contract and eat into future profitability as a result of pressure on product pricing. A number of companies are working hard to create similar fitness-tailored products at lower price points, and Fitbit also faces competition from watches that offer a more diverse set of features that go beyond fitness. Fitbit has done a good job of threading the needle and finding its niche within the fitness-monitoring realm, but it will have to keep executing flawlessly to sustain its past success and pull its stock price back up.

Finally, A. Schulman plunged 25%. The Ohio-based maker of high-performance plastics and resins released its fiscal first-quarter financial results this morning, and Schulman failed to keep pace with its shareholders' expectations. Revenue climbed 5.6%, missing the 9% growth rate that most investors were looking to see, and adjusted earnings of $0.50 per share were more than a dime lower than the consensus forecast among investors. Moreover, Schulman suffered from costs related to its investigation of quality-reporting issues related to its acquisition of engineered-plastics specialist Citadel. CEO Bernard Rzepka noted that Schulman has begun 2016 "on a challenging note, with weakening macroeconomic conditions across several regions, continued pressure in the oil, energy, and material markets, ongoing currency headwinds, and the costs of our internal actions to resolve the Lucent [quality-reporting] matter." Investors clearly agreed, waiting to see whether Schulman's plans to cut costs will be enough to sustain earnings at expected levels in fiscal 2016.