It is impossible to say, by looking at a single photo, if that is a good stone or not or what grade of finish it will produce, each natural stone is different and will perform as such.

I have purchased black stones that look like that, only to find they were white, once cleaned.

There are black and grey Ark stones that are not hard Ark stones use for finishing and there are some that are a combination of Surgical Black and Translucent that are great stones.

Is “ctm”, centimeter? If it is, then it is about an 8 inch stone and if the price is right, worst case you will have a nice stone for knives.
Buying oily, natural stones for razors is a roll of the dice, I have bought many and only a few are great performers.