Ammonite
Pyritized - mother of pearl back - sliced open and polished, 1 1/2 inch
Deshayesites desaysii? or Aconeceras trautscholdi?
Jurassic - 180 mya
Ryazan, Russia
Ammonite has been minerally replaced with Pyrite.

SEE PYRITIZED AMMONITE FOR FULL DESCRIPTION.