re: the expensive japanese stones. They are expensive for various reasons, but quite often, anyone who speaks English or can speak it is just giving us a tug. They're pretty much a sedimentary hone with 15-17% natural aluminum oxide (thus cut stronger than some slates if the binder is right, but generally less strong than high alumina density synthetics). The softer the steel, the less important the cuttings speed is as any SiO2, novaculite, etc. will cut steel that's below 60 hardness.
I bought and sold used japanese stones for a while to undercut dealers and feed my habits at the same time. I have no problem breaking even, but no interest at the same time in having an actual business, so that's what I did. Long story short, I can spot what will be good stones in proxy listings overseas and do a pretty good job with selection, as well as distinguish what's expensive due to rarity vs. what's expensive for use.
https://buyee.jp/item/yahoo/auction/r347452535
This is a stone I bought. The keys in this case are that I can tell from the slurry on it that it's medium hardness (actually very similar to a welsh slate), it's fairly strong cutting and the white is desirable. It's about as fine as a charn, but cuts faster, and there's several lifetimes worth of stone there.
As far as the other stones around, I think (I have bought most, except I've never found a reasonable WOA in large size that's labeled for razors, etc) that it's probably good to stick to convention with domestic/local stones in the UK:
* charnley - great. Varying fineness and softness, but generally good finishers. You guys can find them on the ground for a bob or two, but they're $100 to us here in the states and less good as a tool stone than a trans or good black ark, so that makes not so much sense
* idwalls - fantastic (gray usually with little black dots). Usually a little small, and the good ones are as fine as the finest trans ark stones. Lovely.
* tam-o-shanters - varies from stone to stone, without the label, you can't tell what they were intended for. Most modern woodworkers would probably prefer the finer graded stones
* various welsh slates and other slates (purple, black, dark gray, whatever) - variable, but none are quite as fine as they're boasted to be. A good finish stone, though.
A good slate hone is one where the layers of slate don't delaminate on you with introduction and drying of water, the level of coarseness is something you like, and the level of hardness is high enough that a light touch on clear water doesn't pull particles out.
The sandstones can generally be written off unless you're using something soft like a scythe where you want slow cutting. On a grinding wheel, they're fine as long as they're coarse enough to grind steel well. If they're fine, hard and glaze over - no good. But for medium fine stones, they have been second line stones and are ultra sensitive (not very useful) to steel above a certain level (and it's not a very high level of hardness - even a good stock stanley iron can be problematic).
