This is a Fossil Fern, it is from the Mazon Creek, Ill area and the era is the Paleozoic time frame. The approx. measurements on this piece is 110 x 74 mm: thickness is 17 mm: weight is 280 grams.
 
Mazon Creek fossil beds are a conservation found near Morris, in Grundy County, Illinois. The fossils are preserved in ironstone concretions, formed approximately 309 million years ago in the mid-Pennsylvanian epoch of the Carboniferous period. These concretions frequently preserve both hard and soft tissues of animal and plant materials, as well as many soft-bodied organisms that do not normally fossilize.