Don't know. Not sure that I really care, as these stones definitely did the job that was asked of them, and did it with ease, even for a beginner whetstone user. I'll probably stick with them, at least until I get a lot more experience. Though, I do see a 10,000 or higher grit stone in my immediate future - more to play around with, than a feeling of genuine need.
If you’re sharpening high quality knives, you probably don’t want to use a cheapo sharpening stone. But if you’re just getting started with sharpening your pocket knife, there’s no need to get too fancy right off the bat. You can find a sharpening stone at most hardware stores for about $10. This one is very similar to the one I use. Nothing fancy. Most basic sharpening stones come with two sides: a rough grit and a fine grit. The finer the grit, the finer or sharper you can get your blade. You usually start off sharpening on the rough grit and then finish sharpening it on the finer grit.
You've acquired a good chef's knife, you're using it almost daily to make tasty dinners for the family, and it's stored in a nice knife rack or a magnet for safekeeping. So why stop there? Keeping that knife's edge fine will make cooking not only safer but, let's face it, much more fun. Whether you've spent £150 on a high-end knife or under a tenner on a dinky paring knife, keeping it sharp is crucial. 
One thing about this stone is that it is very "thirsty." I had a cup of water close by just to add a little water periodically when it felt like it was scraping a little too aggressively. Also, be sure to do as the instructions say and use different parts of the sharpening surface. I wasn't paying attention the first few minutes, and apparently I would end each sharpening pass in the exact same spot, creating a very very small notch on the edge of the stone. So I turned the stone around and made sure to vary the ending spot, and it seemed to buff out that little notch very quickly. My knife is now incredibly sharp--it seems to glide through material rather than "cut" it. I know some people would continue to go even higher with the grit, but for my purposes this whetstone is just perfect. I'll be tackling the kitchen knives next!

Water stones can also be made out of natural or synthetic materials and they are fast becoming the most popular type of whetstone as they only require the use of water to lubricate the stone. They are not as messy to work with as an oil stone and deliver fast sharpening results but for even better results, soak the stone in water for 5 or 10 minutes.
J. Kenji López-Alt is the Chief Culinary Advisor of Serious Eats, and author of the James Beard Award-nominated column The Food Lab, where he unravels the science of home cooking. A restaurant-trained chef and former Editor at Cook's Illustrated magazine, his first book, The Food Lab: Better Home Cooking Through Science is a New York Times Best-Seller, the recipient of a James Beard Award, and was named Cookbook of the Year in 2015 by the International Association of Culinary Professionals.

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.
All stones require either water or oil as a lubricant to sharpen the knife. We prefer water stones because they’re easier to use, less messy and don’t have the possibility to go rancid like oil does. If you choose a water stone, all you have to do is either add water to the stone before placing the knife on the surface, or soak the stone in water for 10 minutes before use. You’ll want to read the instruction for the stone you purchase to find out how to use it properly.
Aesthetics – While it’s true that most people keep their sharpener, (even their expensive mechanical sharpeners) in the drawer until it’s time to use them you’ll still want to be aware of whether your sharpener fits into the overall aesthetic of your kitchen when you do take it out to use. While sharpener designs are fairly limited to be sure you typically have some control over the color and finish of the device as well as design factors like whether the device is boxy or rounded in appearance. With a stone sharpener or a stick however you pretty much get what you get.
The Juuma sharpening and honing stones offer a simplified working principle while at the same time ensuring the highest possible quality in the offered grits. Juuma Cobalt Blue stones are made of an aluminium oxide and a bonding agent. Adding cobalt serves to slow stone abrasion and increase the speed of sharpening. The speed bonus is especially marked when stoning blue steel (blue paper steel that is often used for Japanese planes and chisels). The cobalt gives the stones their blue colour. Juuma is our proprietary brand. Juuma sharpening stones are produced by a renowned Japanese whetstone manufacturer.
Understanding Grit: You’ll see a bunch of numbers being thrown around when discussing the grit of a sharpening stone. The number refers to the size of the grit. The larger the number, the finer the grit. A good beginner stone would be a two-sided stone with #400 and #1000 grit. The #400 grit is the courser stone for working out the nicks and imperfections, the #1000 grit is for refining.
Using the strength of industrial high precision superior mono-crystalline diamonds, Diamond Machining Technology (DMT) have created a durable longer-lasting flat sharpening surface that will efficiently sharpen, hone, deburr and polish almost any type of knife or cutting tool. The DMT set contains 3 one-sided whetstones that have extra-fine, fine, and coarse grits which means you have a complete sharpening system at your fingertips.

Water stones can also be made out of natural or synthetic materials and they are fast becoming the most popular type of whetstone as they only require the use of water to lubricate the stone. They are not as messy to work with as an oil stone and deliver fast sharpening results but for even better results, soak the stone in water for 5 or 10 minutes.

using an appropriate blade for the task – a thinner blade for more delicate work, and a thicker blade whenever a thinner blade is not required (e.g. a thinner blade might be used to cut fillets, butterfly steak or roast for stuffing, or perform Mukimono, while a thicker one might be used to slice or chop repeatedly, separate primal cuts of poultry or small game, or scrape and trim fat from meat or hide, as these actions would be more likely to cause unnecessary wear on a thinner blade.)


The coarse stone will cut the metal off quicker but it is going to give you a rougher edge but that way the job gets done quicker, without the oil. It is not as messy. This is just a real simple set up. If you do wood work you can make a little wooden box and rout it out. In this particular case it is just a 2x4, stone traced out, finishing nails tapped down so they are deeper than the stone so when you drop the stone in, if you are at a workbench you can C-clamp it down in place or you can hold on to it.

Siliciclastic stone is a clastic, noncarbonate, sedimentary stone that is almost exclusively silica-bearing and exists as either a form of quartz or, another silicate mineral. In addition, hardened clay is also a sedimentary stone but, it is formed from organic materials such as plant and animal matter and thus, it is much softer than Siliciclastic However, when silicon sediment is suspended in a clay matrix and then naturally hardened over thousands of years, it forms an excellent whetstone material; although, it is somewhat softer than Novaculite. Thus, because the geology of Japan once held large deposits of this type of stone it has been used for hundreds of years for sharpening tools, knives, and swords. However, unlike Novaculite, Belgian Blue, and Coticule, both natural and synthetic Japanese whetstones use water for lubrication and thus, they are commonly known as “Japanese Water Stones” because this type of stone is very porous. Thus, natural Japanese Water Stones must be soaked in water for up to twenty-four hours prior to use whereas, synthetic Japanese Water Stones can be soaked for only a few moments.
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