Sometimes it happens, because the surface is not enough smooth. After 2 may be 3 sharpening times (if you sharp a razor with light pressure + without slurry) the surface will became more smooth and it'll cut not so angry.
About grit range... it depends from concrete stone and actual properties may vary.
Without slurry and with smooth surface mine example works as a razor finisher.. app. 8-10RGrit range or little bit higher.
The plus of this stone is it's softness, but at the same time - without auto slurry. Because of that it may be flatten very fast (softness)+ can be used as a finisher (no auto slurry).
Some of this stones (for me - they looks the same):
A Super RARE Natural 7 1 4 034 Dalmore Blue Hone Stone for Razors to Ect | eBay
Natural Honing Stone | eBay
There were two more of them but i've lost links ,(
Attachment 195834Attachment 195835
Attachment 195836Attachment 195837