I have a new toy coming tomorrow. I'm sort of blurting this out in the middle of this thread while others are having a spat, but I have been through more woodworking stones than I can recall... and though I would call my self "not a user of oilstones", I've probably been through 8 or 10 of them, too, including a hard black ark and two big tranlucents.
Sold them all, and put oil on an 8x3 purple welsh slate - it's a faster cutting hone with smaller but more aggressive particles once it settles in.
But still have a jones for another ark, so I ordered (with the wood shop in mind and not necessarily razors) a "black translucent" arkansas, which I'm going to assume will just be a very very dark translucent, or perhaps maybe it's a very very dense black stone with some translucence.
I'll snap a picture of it if I can remember. What strikes me about it for razors, and why people puzzle over them (it was probably said already) is that they are coarse stones when freshly lapped - they have large particles, and the black arks have some really large random particles in them, from time to time with tools one will let go and give you the sensation that you accidentally left a large stray grit from somewhere else on the stone (you know...the crunching noise and a scratch across the surface, too). I think those particles can be many times larger, like 5 or 6, than the underlying grit size of the stone. Leonard lee, I think, had a picture in his sharpening book of the surface of one - it's alarming, but if they don't ever come loose and the surface of the stone settles, it's like sharpening an airplane wing on golf balls with some soccer balls mixed in.
At any rate, the keys to getting a good edge (and there are two) with an arkansas stone, at least if you want the very finest from them, are:
1) to allow the surface of the stone to burnish itself, while still making sure that it is flushed free of random metal. That means no lapping. The stone will cut excrutiatingly slowly when you get to that point, which means that an edge needs to come with it off of a stone at least as small in grit size, and smaller with deeper grooves (like most of the synthetic 8k+ hones) is ok.
2) to use alloys with either tiny carbides or nearly none. Old simple steel razors should come off of them with a magical edge. Vanadium carbides are probably as hard or harder than a lot of novaculite.
There is probably not a stone harder to get good results from than an ark stone, but once you figure out how to get the results, it's trivial. I know I'm jonesing for mine, even though it'll take at least several weeks of use in the wood shop with high pressure from tools to settle the surface in.