Natural gas prices have been slowly creeping up since bottoming out below $2.00 last April. At $3.64, natural gas is only a few cents off from its 18-month high. And that’s good news for coal investors. As David Steinberg of DLS Capital Management told Barrons , coal prices tend to move inversely with natural gas prices. When natural gas becomes too expensive, some power companies switch to coal – and vice versa. And right now, coal is cheap. The Dow Jones U.S. Coal Index is close to a four-year low right now, and is down 71% in the last year. According to the index, coal has rarely been cheaper over the past 20 years. That makes it a good value play – especially with natural gas prices on the rise. Steinberg recommended stocks such as Arch Coal (NYSE: ACI) and Alpha Natural Resources (NYSE: ANR) as strong value plays for coal investors. My colleague and resident Wyatt Research resource expert Kevin McElroy likes Peabody Energy (NYSE: BTU) , which unlike Alpha Natural pays a dividend. Something Kevin wrote last June in an article titled, “ Buying Coal on Sale ,” still applies now: “I firmly believe that an investment in this space could safely return 100%-200% gains in the next few years,” Kevin wrote. “ The largest coal companies are selling for the cheapest prices we’ve seen since the depths of the financial crisis of 2008-2009.” Nine months later, what Kevin wrote still applies. While coal stocks have fallen even further since last June, as Kevin said, if you look at coal as a long-term investment, you may be glad you bought it at such a steep discount.