Thread: What is it? What should I do with it?

I picked this up today at an antique store. Its very hard and very smooth. It has a large crack in the middle but I think I could still use it if I stayed to one end. Just wanted to have you guys try and identify what kind you think it is. After lapping for 10 min. on 400 w/d, its barely touching it. The minimal amount of slurry coming off of it appears to be a white/gray color. Ill probably have to go to a coarser grit to start. Any info would really help.