From "marqashith" (Arabic) Pyrites
Marcasite is very similar to Pyrites. It is slightly lighter,and the crystals are sometimes spear-like.
Marcasite has the same chemical formula to Pyrites, and looks very similar. The name is used in jewelry, but then it covers true marcasite, pyrites or polished steel.
Larger pictures of Marcasite:
This specimen is natural, showing the needle crystals.
This is a Marcasite nodule which has broken in two.