its a piece of a larger stone... while i was training in japan this year, we were playing around with bits of random things. A large wheel had recently broken into 3 pieces. I got one, another knife maker got one, and the people i was training with kept the rest. Turns out it makes a pretty solid and fast cutting medium grit stone if you use the side
The making, use, maintenance, repair, etc., of these kinds of stones is not as simple as most people seem to think (i get lots of e-mails asking where people can buy one). Craftsmen build their own setups, and the stones can vary in type and construction depending on what they are looking to do (and preference). Some are tempermental to temperature, some cant be soaked (but are ok with water being splashed on them), some only work with certain types of steels, etc. Also, the keeping flat can be odd if you've never seen it before. It can also be a source of great damage if not done properly.
Wow that is a nice size hunk to ship from Japan. Unless you put it in your carry-on....lol. Hope it sharpens many knives in your capable hands. Or are you going to cut it down to make many smaller stones?