Natural stones are graded from their density rather than their grit, making them inconsistent compared to their synthetic counterparts.
Natural water stones require natural nagura stones to raise a slurry in order to manipulate grittiness. This is also a characteristic that differentiates naturals from synthetics which require synthetic nagura stones for cleaning, not slurry formation.
Natural stones are mined and require a very experienced eye in order to grade them. That being said, natural stones bring a whole new level of sharpness and retention to your knives.