You need the right size of sharpening stone that fit the type and size of your blades and tools. In general, 6-inches stones are the smaller models and 8-inches are the larger ones. The stones larger than 8 inches are work best for big-sized blades and tools. You may also find sort of ‘pocket knife sharpener’ out there. Most of the credit card sized sharpening stone are smaller than 6 inches and ideal for on the go sharpening not for regular basis.

In most cases a sharpening stone will be a combination of sharpening grains and a binding agent. However, in case of a Ardennes Coticule it is a completely natural product. The grains have cutting edges which enables them to sharpen your knife. As soon as a sharpening stone is used little pieces of the grains break off, revealing a new cut ing edge. The higher the number, the finer the grain. Stones with coarse grains (up to grain 400) can be used to shape the blade of a blunt knife. You can subsequently take care of the fine finish with a stone with a smaller grain.
Both Belgian Blue and Vielsalm Coticule are ancient stone layers found in the Belgian Ardenne Mountains with characteristics similar to both Novaculite and Siliciclastic sedimentary stone in that it is a metamorphic stone consisting of both gray and yellow volcanic ash mixed with tiny Spessartite Garnet crystals suspended in a clay matrix. However, due to its geology, both types of stone occur only in vertical seams sandwiched between two thick layers of bluish-purple slate and thus, they must be meticulously extracted mostly by hand. However, this type of extraction process is both very time-consuming and very labor-intensive and, quarrymen can only extract the stone for a few months each year due to inclement weather conditions. Consequently, both Belgium Blue and Coticule whetstones tend to be somewhat expensive.
In summary: Peter Nowlan is a professional knife sharpener based in Halifax (Canada) and he recommends the KnifePlanet Sharpening Stone Set, a beginners and intermediate kit that includes 4 sharpening grits: 400/1000, 3000/8000, a bamboo base and the KnifePlanet Flattening Stone. The Japanese Naniwa 3-stone combination is also a great (and more expensive) choice, ideal for professionals and more advanced sharpeners: the Naniwa stones are slightly bigger compared to KnifePlanet’s. In both cases, a coarse, medium and fine grit combination is very effective to sharpen and refine the edge:
I bet those Vikings, relatives of mine no doubt, never had it so good. They never had surated blades to sharpen or needed to wear cool things around their necks to woo the women (they just grabbed them by the hair I guess). I love my stone. So useful and beautiful. I always have spit to wet it with, and I always have a way to sharpen my tool. Wazoo makes high quality products. I give them the highest rating I have. Dudes!
At this point, I can flip the stone over and use the fine course side. Most Whetstones come with a nice wooden base that do a good job to hold the stone in place while sharpening. Before starting, you might need to add a bit more water to the surface. Now with the fine course stone, finish the sharpening process like before at the same blade angle and nice consistent passes along the stone. About 10 to 12 passes is good before switching to the other side of the blade. At this point, the blade should be extremely sharp. I’ll just give a quick paper test here. I can now slice paper quite easily with my knife.

Oil Stones differ from water stones in that they require the use of honing oil to float the swarf (metal particles). Also, these stones are commonly made from one of three different materials consisting of Novaculite, Aluminum Oxide (Corundum), or Silicon Carbide (Carborundum) and they all use oil to lubricate the stone and suspend the swarf in order to prevent the stone from becoming clogged. Also, while Novaculite and Coticule are the most traditional types of oil stones, there are also synthetic oil stones made by the Norton abrasives company called “India Oil Stones”.

However, due to limited natural deposits and extensive quarrying over the centuries, true, high quality, Japanese water stones are now few and far between and are very expensive. Thus, various companies are now producing synthetic Japanese Water Stones which generally have a more consistent composition and grit size than natural stones as well as often begin somewhat harder and thus, lasting a bit longer; not to mention being cheaper to purchase.
Now move the blade – with a little pressure – in regular movements up and down along the sharpening stone. Always maintain the angle between the blade and stone. You will notice a burr become visible after five or so movements. Mentally divide the blade into three sections if the knife has a large blade. Always start with the tip and work back towards the bolster.
The Japanese sharpening stone whetstone combo is about 9.1 in. in length. It's  multi-sided and actually looks like a block of wood. When you use the dark side or the medium grit side (#1000), soak it for about 2-3 minutes.  When finished, the ridge on the base which surrounds the stone will fit snugged into it. Even if the base wasn't there the stones friction will stay. Some of the good features of the base is actually the rubber stoppers to hold it on a dry surface. When you learn how to sharpen a pocket knife, doing it on one of these will help.
There are no lengthy explanations needed to describe the Lansky 8” Ceramic Sharp Stick sharpener. It’s an old-fashioned device comprised of a hardwood handle and ceramic sharpening rod. That’s it. Sure it’s not going to create an absolutely picture perfect edge down to the last micron but it will keep your knives really sharp and do so for next to nothing.
I've been using this stone for months and have started to find that, while the fine side does provide a nice sharp edge, the 400gr side is wearing my knives unevenly, which causes me to be unable to finely sharpen the entire length of the blade when moving to the fine side. The logo that is put in the middle of the 400gr side of the knife is causing more material to be removed than the rest of the blade when I'm sweeping across. I even verified this by going straight down the length of the fine side and watching the wear pattern come from the material coming off of the knife onto the stone.
Your stroke can be straight or circular, from "hilt to tip" OR "tip to hilt," whichever is more comfortable. If you're using a small portable sharpener, stroke the blade in nearly a straight direction. Remember to always cut into the stone and never pull or drag your edge backwards. The blade edge should face in the same direction as your stroke. So, you're essentially moving the metal away from the edge. We recommend the circular stroke as it helps you maintain your angle instead of having to find it every time you lift the knife from the stone.
I would say they are worth the money. Go for it. I took a blunted Pampered Chef kitchen knife from completely dull to razor sharp in about an hour (of bumbling and repairing mistakes LOL) heavy grinding. If you have a severely damaged blade, grab one of those cheesey two sided stones from Harbor Freight to do your heavy grinding. From there these stones will work very well.
Chefs will do this every day, and there's no reason you shouldn't too. Before cooking, or after you've done the washing up, honing your knife will help keep it in good condition. "When you're using a honing steel, you're not actually removing any metal at all, just re-straightening that edge, to get it back in line," says Authbert. Remember that you'll still need to sharpen it every two or three months. 
A: That depends almost entirely on how often you use them. If you are a professional chef who uses his or her knives every day then you should hone them every time you use your knives. But honing has its limits and over time your edge is going to become dull no matter what. Therefore your knives will need to be sharpened at least several times a year. Some chefs, in fact, will use a whetstone on an almost daily basis in order to ensure their knives are always razor sharp. Most of us, however, are not professional chefs and may not even touch our kitchen knives for days at a time. In that case, it’s probably a good idea to hone the edge after every couple of uses and have your knives sharpened once a year.