I took a diamond saw and cut the rock into that was pulled out of a drained pond on a farm in Indiana. It was messy experience with the surface of the rock coming off and getting all over me. The water from the wet saw as spraying everywhere.
The rock appears to be pyrite. It was dull looking when I cut it so I started polishing it with 800, 1000 and 2000 grit wet sandpaper.