A very rare specimen of pyrite having replaced gastropod shell (snail like creatures) from the famous Colorado location known for ammonite fossils. Fossils of other critters here are few and far between, and perhaps 1 in 200 of all found over the years. This is a sharp, complete gastropod shell with small hillocks of secondary pyrite crystallization upon it, and is a perfect and unique thumbnail specimen! These formed from simple chemical processes over time as with several other famous finds of mineralized fossils, given the right conditions in the ground as the soft mud turns to soft stone and then hard stone that entombed the pyrite within: decaying organic matter (the Baculite) provided the Sulfur necessary to react with iron in the environment (likely water rich in iron trickling down when the rock was still porous) and caused iron sulfides (Pyrite) to precipitate and force their way out into the stone as they grew, followed by the whole thing being preserved in rock. Collected by amateur paleontologist and mineral preparator Bill Hawes.