The answer to the question of how to choose a single whetstone from among the many is actually one of both purpose and expense. Indeed, as the old adage says: “You get what you pay for” – This is certainly the case with sharpening stones. In fact, as a general rule, a high quality, natural, whetstone is significantly more expensive than a man-made whetstone but, they also tend to produce a noticeably finer edge than man-made stones do. On the other hand, man-made whetstones (with the exception of diamond hones) are very affordable and, just like natural whetstones, they too are available in different grits for different stages in the sharpening process from cutting the initial edge bevel to polishing a finely honed edge.
The first step to choosing a whetstone is to determine your intended purpose and then choose your whetstone accordingly. For instance, when sharpening tools that do not require a fine edge, you should choose a relatively soft, coarse, stone such as Crystolon water stone. However, for sharpening tools that do require a fine edge, a somewhat harder India oil stone would be a good choice. But, for sharpening hunting knives where an exceptionally fine edge is required, a Novaculite or Coticule oil stone would be the best but, most expensive, choice. So, the process of choosing the correct whetstone for any given purpose is to first determine how fast you would like for the stone to cut and how fine an edge you need, and then choose either a soft, coarse, stone or, a hard, fine, stone of the appropriate type and grit.