Review By Dean Bielanowski  DMT Website -

DMT W250FNCB 10" DuoSharp
Diamond Bench Stone


By Dean Bielanowski

It is commonly known that sharp tools are in fact safer tools. This is because when you are using a sharp tool there is generally less effort and force involved to cut or drill something, and this generally results in fewer accidents as a result. But getting your tools into a condition that you could call "sharp" can be tough without the right equipment. After reviewing the Veritas Mk.II Honing Guide recently on this site, I found a need for a larger diamond stone than the one I currently had. So I searched the various diamond stone manufacturers and eventually settled on the DMT 10" stone I will review here. It was large enough and wide enough for my sharpening needs and does seem to have favorable reviews from existing owners. 

After having used this stone for about 6 weeks now, I believe I have enough qualitative data and results to form this review, so let's get to it!

The DMT DuoSharp Diamond Stone
So I purchased the W250FNCB model DuoSharp stone from At the same time I also ordered the special base that is available for these "stones", also made by DMT. The stone, as its title suggests is made from diamonds, which are known to be one of the hardest materials available to us at this time (researchers have now discovered a harder natural material and have constructed harder materials in the lab, however these are not readily accessible or easily manufactured etc). There are other manufacturers that make diamond sharpening stones. Perhaps the most well known is Eze-Lap, and we have reviewed some of their products here on the site previously. They are very good in their own right too. Using a diamond surface has obvious advantages over other softer materials we commonly find in sharpening stones; aluminum oxide, natural stone and ceramic etc.

Firstly, because diamonds are significantly harder than any tool steel we will use on them, there is going to be very little wear of the diamond surface of the sharpening stone (or at least very slow wear which will see a diamond stone last many years of heavy use). This has big implications when it comes to sharpening stones. The result of this ultra-hard diamond surface is virtually zero pitting, hollowing or grooving as compared to softer sharpening materials. You do not have to worry about moving the blade or tool being sharpened evenly across the stone to reduce the chance of pitting or hollowing occurring in one area, nor do you have to spend time flattening a stone that may have developed some of those problems. Secondly, because of diamond's hardness, you can sharpen a tool much faster than you can with other materials and with less force or pressure exerted. A couple passes on a diamond stone might be the equivalent of a couple dozen on other stones. Third, it is not essential to use a lubricant or fluid when sharpening on a diamond stone, although it does help remove the swarf or material abraded from the tool surface. Not a lot, if any will be abraded from the diamond stone itself. This particular brand of diamond stone has a design that helps remove debris, which we will look at shortly. So you can say goodbye to messy oils and the smells they produce. You can clean this product with water, or use water during the sharpening process if you wish.

DMT Stone Construction
The folks over at DMT make some claims as to their products description, all of which would appear true after having used the stones for some time now. To begin with, DMT uses monocrystalline diamonds in their product, as opposed to the more inferior polycrystalline types found in stones of less quality or price. What's the difference? These two pictures certainly help explain it...

mono vs polycrystalline structure

As you can see, monocrystalline diamonds are larger, single structure pieces, as opposed to polycrystalline which is a diamond structure piece made up of multiple, smaller diamond fragments that are bonded together. You can imagine what might occur to the polycrystalline diamonds over time and use. Their bondings make break down, leading to fragmentation and a "worn" surface. Monocrystalline on the other hand would only wear or "fail" significantly if the hole diamond piece is dislodged from its holding surface, which is much less likely. DMT uses the monocrystalline diamonds in its products, and while I cannot verify these are indeed better in the long run (you would need a pretty good microscope), the logic makes sense. The diamonds are imbedded approximately two thirds of their height into the nickel bed. This provides an excellent hold on the individual diamond stone while still leaving plenty of diamond surface above the nickel bed for sharpening purposes.

DMT's interrupted surfaceThe diamonds themselves are bonded to a machined, flat steel plate within a bed of nickel. Underneath this steel plate is the bulk core of the stone, constructed from hardened plastic. For models which have two grit types, like the one reviewed here, a second diamond layer is found on the reverse side, with the plastic core sandwiched in between. For this stone, DMT offers their "interrupted" surface construction. These are the small round "pits" (recessed dots) you can see throughout the sharpening surface. These are added primarily to collect abraded metal created during the sharpening process and to remove and contain it away from the diamond surface. This way the diamond surface does not "clog" with metal and require constant cleaning or re-abrading of the surface for it to continue functioning (like you have to do on a high speed dry grinder when metal becomes imbedded in the wheel). Cleaning couldn't be easier. Just wash the stone down with clean water and it is ready to go again for next time.

Multi-Surface Stone
This particular model DMT stone, the W250FNCB offers two sharpening surfaces. These are color coded to indicate the grit rating of each surface. DMT uses their own color coding chart, and this particular stone is coded blue for one side and red for the other. Small round color dots can be found on each side of the stone to quickly identify which grit surface you are using. The blue coded surface indicates a grit/mesh rating equivalent to 325 mesh (45 micron). This surface is for "quickly sharpening a neglected edge". On the other side of the stone is the red dot indicated surface which is equivalent to 600 mesh (25 micron). This is to put a "keen edge on a maintained tool". Duosharp bench stones are also available offering a coarser (220 mesh / 60 micron) surface indicated by a black dot, or a finer (1200 mesh / 9 micron) surface indicated by a green dot. Other DMT products (not DuoSharp stones) are available with coarse 120 mesh (silver), and finer 2200 mesh (white) and 8000 mesh (tan) surfaces.

So the W250FNCB model here has the 325 and 600 mesh surfaces... A good compromise between coarse initial sharpening requirements on a pre-formed edge, while offering a more refined surface by using the 600 mesh surface on the reverse side. Different combinations of surfaces are also available in the DuoSharp line.

In Use
Since I have used diamond stones before, I had some idea of what to expect. It is commonly known, or at least complained about in various forums, that the person's new diamond stone doesn't seem to sharpen as fast as it did the first few times they used it. This is NORMAL folks! Look at the image of the diamonds again above. Notice the sharp top points or tips of each diamond crystal. On new stones these sharp tips give a rough look and surface feel to the stone. You can imagine that these sharp tips will round over fairly quickly for the first few uses, especially since near the top of each tip there is very little material beneath it. As these tips wear down, sharpening seems to slow down, and it certainly does slow down, but this is normal for a new diamond stone. There is a certain "breaking in" period where the very top tips of each crystal are worn down. Sharpening slows a little, and the stone actually becomes smoother as a result (better for honing anyway) but sharpening is still much faster than with other stones because of the diamond's hardness. So with this in mind, I reserved making any judgement on this product until I had put the stone through a break-in period (usually only about half a dozen uses or so it seems).

Now, a quick U-turn on the flow. Back to the stone holder I mentioned earlier, called the "DuoBase". This basically holds the DMT stone up off the bench (knuckle saving) in its cradle and with its included eight rubber feet underneath, it goes a long way to preventing the stone slipping on the bench as you use it. The one DuoBase will accommodate all DuoSharp line stones. On the reverse side is a handle. I thought, great, I can carry it around a little easier, but then wondered why you would need a handle to carry a fairly light sharpening stone anyway? It turns out that this handle is also a way to hold the stone for running it over a large surface that may be immobile or not easily manipulated. I haven't really found a need to use it this way as yet, perhaps sharpening mower blades while still installed on the mower or blades on a much larger cutting machine? Whatever the use, the DuoBase is a handy accessory if you don't have any other method to secure the stone and it is reasonably priced too, all things considered. 

The DMT DuoBase with the W250FNCB Stone in place

Now, back to the stone and sharpening. I have been primarily using it for sharpening my hand chisels and bench plane blades. WEith a width of 2 5/8" (67mm) it is capable of sharpening pretty much all my hand plane blades and chisels. The extra width makes things easier when it comes to sharpening. These is nothing worse than trying to balance a large/wide hand plane blade on a narrow sharpening stone and constantly having it slide off the edge as you try to move the blade evenly across a the stone. No such problems here. The extra width is great for use of sharpening jigs that have a wide roller wheel too, like the Veritas Mk. II jig. It also has a generous length of 8" (203mm) providing enough room to really put in nice, smooth, and long sharpening strokes. The stone is 3/8" (9.5mm) thick, but that doesn't really have much effect on sharpening, only to ensure there is no flex in the stone itself, which there isn't.

The one issue with this stone, especially with the recessed dot design, is that it is not terribly suited to tools that have a small or sharp point, like those found on small carving tools or small lathe gouges. These points can drop into the recessed dot areas and prevent a smooth pass over the diamond surface, so this particular stone cannot "do it all" so to speak. DMT do make a DuoSharp stone with a small continuous surface of diamond at one end of the stone to accommodate these smaller/pointed tools. Model numbers for those are WM8EF and WM8FC. But since I havent used or tested those I will not comment further.

Ok, the actual sharpening... The only real way I can tell you how good, or otherwise, this product is, it to explain the speed at which something is sharpened. We all know it is possible to sharpen a tool equally well or to the same degree of "sharpness" on a diamond stone as it is on a refined aluminium oxide or other type of "softer" stone. The difference is in the speed, or time it takes to get there. The results I give you are only qualitative. I is hard to setup an experiment for detailed testing without the use of multiple machines, and then there are many variables that can affect the outcome. My simple test was to take two chisels of the same brand, same size and type. Both were in need of sharpening. So first, I put them on the dry grinder just to rough grind the bevel so each chisel was fairly similar in its surface grind (as far as I could tell by eye anyway). Then I went to work with the DMT DuoSharp stone, using it on one of the chisels, while sharpening the other chisel on a basic, standard aluminum oxide stone (with two grit surfaces as well) with each surface probably even slightly coarser than the diamond stone. I used water as the carrier for the DuoSharp, and oil for the AO stone and sharpened both chisels using the Vertias Mk II honing guide.

Without babbling on too much... I refined each chisel to a similar looking edge. The results... About 25 minutes of work on the Al Oxide stone... About 5 minutes of work on the DMT. This was total time, not actual sharpening only time, but as you can see, there is a large difference in the times to refine each chisel to a similar edge, starting from a similar point. There is no doubt that Diamonds are able to abrade a surface much quicker, and with less effort than other "softer" sharpening stones. This does not account for the cleanup time required for the al oxide stone, or any time to resurface it later down the track either. I know which one I would be reaching for!

Well, we probably already know that Diamond stones work faster to sharpen our woodworking tools. And yes, to get them ultra sharp like some of us like them, they will need further refinement after sharpening on this stone. I have a leather wheel and diamond paste which quickly takes care of that, among other sharpening contraptions! But perhaps you wish to know how it compares against other diamond stones? Long story short, forget all those budget diamond stones or sharpening sticks you see being sold here and there for next to nothing. They are often poorly made with low quality, polycrystalline diamonds and don't do a particularly good job. There is a marked difference between the lower end and higher end of the diamond stone market in terms of quality and sharpening results. Against another quality diamond stone product, the Eze-lap, I would say the DMT has it beat, but only slightly. Both stones produce great results, but the Eze-lap doesn't have the recessed dots for clearing material away from the surface. You can see it building up, which no doubt slows the sharpening down, if only marginally. On the other hand, the Eze-lap's continuous diamond surface allows it to be used with small and pointed tools, something which this particular DMT model stone does not. Both are great products, so think about what you intend to sharpen on the diamond stone before you decide which to go for. But if you have the money, this particular DMT stone (it retails for around US$100 - Feb '09) is well worth it in my humble opinion. 

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DMT W250FNCB Photos
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The W250FNCB, with its coarse face showing, as indicated by the blue dot.

The reverse side of the stone with the fine face showing, indicated by the red dot.

Closeup of the coarse surface with the recessed dots for holding swarf and abraded material.

Sharpening a small chisel with bevel down.
No water used here on the stone.

With water added to the stone, the swarf is more easily removed from the diamond surface. the heavier particles find their way into the recessed dots while the lighter particles become suspended in the water.

Yes you can flatten the back of the chisel too quite easily with this diamond stone.

Cleanup couldn't be easier. Just wash it down with water and it is almost like new again!

A shot of a bevel sharpened on the DMT W250FNCB diamond stone. This chisel is now very useable, but for the ultimate in sharpness, more finer honing and polishing will be required.

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