What's the difference between a soft and hard Arkansas stone

[TheMonsterTruk]

I bought a W.R. Case and Sons Soft Arkansas stone a while ago. It works amazingly but I heard something about Hard Arkansas stone. What's the difference between the two? Is it a grit thing, or what?

 

[hardheart]

density. all arks are novaculite, and the novaculite is all pretty close in abrasive size, as it is composed of the skeletal remains of radiolarians.

 

[ryxlyx]

"Hard" stones finer grain than Soft, "Black Hard" finer still.