Looks like tumbled travertine-creosote is the by product of a wood burn. (I think there are many variations of creosotes)
I have done a few of these-Prosoco makes some good cleaners like Fred said.
They have a fireplace cleaner that works good-most high alkaline cleaners will do OK though.
However I have found the best method for removing most of the black is sand it off.
Use 80 grit sandpaper or 70 grit electroplated and cut right though it.
Be mindful this is a tumbled stone with rounded edges.
Then you can use cleaners and or poultices to get off any that may be left.
Usually there isn't much left.
Once you get it cleaned off the next step is to restore the tumbled look.
So we use diamond or carbon brushes to replicate the tumbled look or as close as we can get it.
80 or lower honing powder will help as well and then you can finish with diamond impregnated pads or similar.
Go give it a test with some sandpaper and see what you get-That's looks like a good job to take on-