Rinsing with water works fine. Just feel the stone thoroughly to make sure there aren't any higher grit particles on it, and while wet, look at it in the light across the surface at eye level - any particles will cause the water to swell up around them so you can see them. As for the natural stone, price is too random to bother quoting anything. Talk to a reputable vendor (I bought mine from Maksim at
www.japanesenaturalstones.com), let them know what you are looking for, and make sure that whatever else they've used and know the quality of any stone they are selling. Price will vary wildly based on where the stone is from, how hard it is, what the size is, what the composition is, etc.