A sharpening stone is made of particles of abrasive material that are sintered or bonded together. The blade is moved across the stone and the steel is worn away, which creates the edge. However, at the same time, the stone is also worn away to reveal new, coarse particles. As a general rule, the softer the stone, the more rapidly it will wear and will be more aggressive in use. Harder stones don’t wear as quickly.
Whichever method you choose, be it a waterstone (also known as a whetstone), or a pull-through (either V-shaped or ceramic wheels) it's important to regularly hone your knife with a honing steel, which we'll also cover below. You'll be pleased to hear that you won't have to reach for the stones too regularly – once every two or three months should suffice.
Now you may wonder, if a finer stone produces a better cutting edge why do I need a coarse stone at all? The answer is that finer stones work much more slowly than coarse ones. The amount of metal a fine stone removes with each pass is much less than what a coarse stone removes. If you have an edge with a good shape to it from a coarse stone, this is not a problem because the fine stone doesn’t have to remove much metal to improve the edge. If the edge is dull, it will take many times longer to reshape it with a fine stone than it would with a coarse one.