May's highly anticipated employment report, courtesy of the U.S. Labor Department, came out before markets opened this morning. With nonfarm payrolls up by 217,000 last month, and overall employment reaching a new all-time high, there was a lot to like. Of course, the 6.3% unemployment rate is still a concern, but why fret over the fine print? As the S&P 500 Index (SNPINDEX:^GSPC) ended the week at record highs, Cliffs Natural Resources (NYSE:CLF), First Solar (NASDAQ:FSLR), and Peabody Energy (OTC:BTU) each lost ground, finishing near the bottom of the index. The S&P 500, for its part, added about 9 points, or 0.5%, to end at 1,949.

Cliffs Natural Resources was the worst-performing stock in the S&P today, shedding 2.2% as it capped off a week during which shares fell 7.7%. The fact that iron ore prices are currently low in and of itself is a setback for Cliffs Natural Resources shareholders, but increasing scrutiny from analysts and one loud activist investor represent further headaches. Research firm Cowen group noted that, as long as iron ore prices remain depressed, Cliffs stock will, too, and the business could struggle to even remain profitable. Meanwhile, hedge fund Casablanca Capital wants to see heads roll on the board of directors, and Cliffs is trying to stem the bleeding by investing less in its business. Occasionally, stocks fall for no good reason. Occasionally, a stock's slump makes a lot of sense.

Shares of First Solar lost 1.8% Friday, and for no good reason. The stock enjoyed a nice rally on Wednesday after the U.S. Commerce Department announced new tariffs on Chinese solar manufacturers that should benefit the U.S.-based First Solar. Ranging from fees between 18% and 35%, the move combats foreign competition in an industry where Chinese manufacturers, given subsidies by their government, have been illegally "dumping" their product in the U.S. at losses in an attempt to gain market share. The increased tariffs are good news for First Solar investors, but that good news was quickly forgotten by Wall Street.

Peabody's coal mining operations in action. Source: Peabody Energy website

Lastly, shares of coal miner Peabody Energy were down 1.5% today, largely because of a downgrade at the hands of Goldman Sachs. Goldman's opinion -- rightly or wrongly -- carries a lot of weight on Wall Street, and with the investment bank slashing its price target (from $21 a share to $16 a share) and rating (from buy to neutral), investors were spooked. Goldman's reasoning was sound, as lower international coal prices, weakening demand from China, and burgeoning competition in Australia combine to create more challenging economics for Peabody Energy. With that in mind, individual investors should always take analyst opinions with a grain of salt, since their interests don't primarily include the growth of your portfolio.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.