Experience, that’s a soft Washita stone.
And No, natural stones cannot be grit rated because they are natural an each may/will be different. You can grit range your stones by honing half a razor with a known stone, say 1k and comparing the pattern to your stone on the other half.
It may not tell you the grit, but will tell you where it can fit in a progression. You probably could set a bevel on it, but is better for knives that require a lower grit, sub 1k.
Best and easiest to start honing with a known good for razors Synthetic Hones.
