Every Japanese natural is different. You need to rely on the seller's description of the specific stone that you are considering, and be sure that you have return privileges.

I feel that many of the hard grey stones currently sold as Ozuku probably are not, but I have tested a few and if you have a good one, it will likely give you an improvement in your edges.

There are also other options like the Shobudani type 100 which Max and Alex both sell or the equivalent of it. Type 100 is just a size BTW. Takeda-San used to sell Nakayama koppas (irregularly shaped stones) for around $90 and these were also good, but I don't know if he sells them anymore.

Good luck!

Cheers, Steve