St. Clair, Pennsylvania(United States) fern leaf plant fossil slabs for sale. Educational, affordable, cheap, and authentic, these natural Carboniferous fern plants died over 300 million years ago. A much needed addition to any rockhound's rock, fossil, or mineral collection. St. Clair plant fossils are also a valuable resource for studying, collecting, or as a geology, paleobotany, palaeobotany, paleontology teacher's aid.