Help to I.D. hone

[baldy]

I picked this up at a garage sale today for $4. I have no idea what it is but I was just curious and for $4 you cant really loose.
I lapped it and cleaned up the edges. I then gave a completely blunt blade 15 laps over it with almost no effect on the blade, but thats all I've done with it so far.
It seems quite hard and fine and if you look at it on the right angle you can see fine sparkles in the surface.
It also did seem to develop a light grey slurry when lapping. Its 2" wide by 1" thick.
The pics below are before and after lapping
Thanks Grant