Fellwar Stone was recently reprinted in Commander 2015, which caused its introduction to a whole new generation of Magic players, while hardly changing the availability (it's an uncommon from several old, heavily printed sets). The original The Dark printing has started spike-percolating, but I think its yet to really pop.
You can still find these around $1.00 apiece, under rocks and at the bottom of boxes. Spend up to $2.00 each right now for black-bordered, original printings. Act fast, these are an easy $5.00 apiece within a couple weeks. I can see the original copy sitting around $10-$15 in a year or two. 9th Edition Foils popped over a year ago (it is the ONLY foil printing), and are due for a second price hike within a season or two. Scoop up 5th, 4th and especially 9th Edition copies under $0.50 apiece as long-term, slow-gaining sure-bets.
This price movement is due to cumulative Commander demand finally breaking the back of the original printing.