I travel a little and as I do, I pick up stones that look like they have sharpening potential, take them home, flatten a spot and see how they sharpen. Most are duds, but I did find one in South Carolina earlier this year in the 3K grit category that is seriously as good as any of the JNats I have.
To me, I don't think natural sharpen stones are exclusive to Japan, but........ there is something exotic in the foreign...... A particular JNat I love is a Meara. When I hit it with the slurry stone, it emits these fantastic odors which makes my mind create all sorts of delightful fantasies about the mountains and rain forests in far away lands. Now compare that to the stone from South Carolina. When I hit it with the slurry stone, I have delusions of banjo music and running in terror
Yup, I think that is why Japanese stones are more sort after.