Naturals don't have grit ratings, though the finish off a natural can be said to be similar to "synthetic hone X, which has a grit rating of Y." Many will also say "natural stone Z is like a Q grit hone" without comparing to a specific, known synthetic. How you personally take statements such as that is up to you.

Two of the more popular high grit natural choices are Thuringians (specifically from the Escher brand), and Japanese natural finishing stones. To get a good Japanese stone, you need to buy from someone you trust, because they are all different and there can be huge variations.