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.
It is believed that Vikings suspended these knife sharpening tools from their belt. However, it is not exactly clear, even to scholars, how the Vikings wore these pendants. We laced ours with a versatile, adjustable high-quality leather cord and packed it in a USA-made muslin pouch so you get to choose how to wear it or carry it. Wear it around the neck for the most Ragnariest looking necklace you’ve ever seen, cinch it all the way down so that it can be hitched to your belt or clipped onto your pack, or simply pocket it while still contained in the hand-stamped muslin pouch.
Modern synthetic stones are generally of equal quality to natural stones, and are often considered superior in sharpening performance due to consistency of particle size and control over the properties of the stones. For example, the proportional content of abrasive particles as opposed to base or "binder" materials can be controlled to make the stone cut faster or slower, as desired.[7] Natural stones are often prized for their natural beauty as stones and their rarity, adding value as collectors' items. Furthermore, each natural stone is different, and there are rare natural stones that contain abrasive particles in grit sizes finer than are currently available in artificial stones.[citation needed]
But it's a different type of sharp, according to Joe Authbert, product development manager at ProCook. "What it does is add tiny little micro-serrations onto the edge of the blade." But fear not - your smart knife won't end up looking like a bread knife, as you'll be hard-pressed to spot the serrations. "If you looked at it under a microscope, on the cutting edge, there are these little lines that generate the sharpness, rather than a waterstone which is a smooth sharp edge," says Authbert. 
I bought this and Unimi's 600/1000 whetstone at the same time. Curiously, reviews are currently blocked for their 600/1000 model, as amazon wouldn't let me leave one for it. Considering the conspicuous absence of reviews in the other stone's listing, one can only assume they're being censored for everyone. So, maybe I should be more critical of these stones, and use them a while longer before giving them any praise? But I've had a very positive experience with them so far, and the 600/1000 stone was the one I found most useful.
I recommend keeping two stones in your kit. One with a medium grit (around 800 or so) to perform major sharpening jobs, and one with a fine grit (at least 2,000) to tune the edge to a razor-sharp finish. For real pros, a stone with an ultra-fine grit (8,000 and above) will leave a mirror-like finish on your blade, but most cooks won't notice the difference in terms of cutting ability.
The video above, from the How To You YouTube channel, explains everything you need to sharpen with a whetstone at home, and the best methods for doing so. Obviously, you’ll need a whetstone (as well as a few other things), and you’ll want to work near your sink or a bucket so you have easy access to water. Fill a container with water, then soak the whetstone in it until it stops bubbling. Inspect your blades for any trouble spots, and get to sharpening using the methods and motions described in the video; making sure to check your progress as you go. We’ve talked about sharpening knives with whetstones before, but this guide is a bit more thorough and the old video has since been removed.
Not sure why I bought this item and then I received it. It has a bit of a 'cool' factor and is well made. I took the sharp edges of the stone as they were a bit uncomfortable on my chest but have since been wearing it a lot. Used it to put an edge on some workmates knives and to tickle up my own daily use knife. I like it, it is a bit different and useful.
first time buying a sharping stone. i bought this because i wanted to sharpen kitchen knives. you have to practice to get good at it, use a knife you don't care about because you might mess it up. there is some conflicting instructions online on how to use, water or no water. read as much as you can about using stones and practice, practice , practice. look up videos on youtube and internet. i had fun using it, BUY IF YOU WANT TO LEARN here is some stuff i found, they all sound like professionals http://video.about.com/culinaryarts/Sharpen-Knives-With-a-Whetston.htm?rd=1 http://culinaryarts.about.com/od/knivescutlery/ht/whetstone.htm this is a video that says don't use water http://video.about.com/culinaryarts/Sharpen-Knives-With-a-Whetston.htm?rd=1 this is a youtube video that i watched that uses water. *informative http://www.youtube.com/watch?v=KFIg9Cm-nJg