I only just saw this thread now, I just off loaded a few recently, still deciding about my last ones.
They are special because only hones from the Nakayama and Shoubudani quarries are the genuine Honyama hones. The term Honyama has now been bastardized to mean finishing stone.
Be very cautious of any other website you find selling Japanese stones because a lot of them are claiming to have such stones have no proof of what type of stones they are selling. At the ridiculous prices I have seen them selling for, I'd like proof, especially when some look like a person went to their garden and dug up a few rocks and say they are Japanese.
Give So a try though, he comes quite recommended here.
JMHO.