This is part of a series of blog posts – looking into the appearance and composition of commercially available sharpening stones. If you are interested in the previous episodes, check out the archive for them.

If you have some suggestion on what I should look at next, or want to share your super secret DIY stones, I could be persuaded to open the bag of analytical devices… hit me up on Instagram under @marvgro for that.

Disclaimer: I’m not for sale. Every review you see on this blog is bought with my own money. I have no affiliation to any manufacturer. This segment features a review on my own product, so it can be considered an advertisement in some countries. You are warned!

Review

Today’s sharpening stone is something I didn’t want to do. Like, really! I found what I considered the perfect solution for re-setting and even creating the bevel. The fantastic ATOMA F400 sharpening stone was my go-to-resource for setting the bevel and completely regrinding a blade. You might notice on my use of the past tense – it’s been replaced. And, to backtrace – not by something I particularly wanted to do – but I got asked a lot, so I tried and engineered and found a solution – proudly presenting the latest addition, the Dr. Marv Ultracoarse Scientific Sharpening Stones!

I firmly believe that the correct “jump” in terms of grit progression is to half the size. While it is possible to do a larger jump, the time spend to remove the scratches or damages from the previous time often make using another stone quicker. Moreover, when the sharpening action on one stone takes takes too long, one is liable to press harder. With my current lineup of stones ranging from 80 to 0.1 µm, the logical choice was to choose something along the 150-160 µm range and another, even coarser stone at double that value. Let’s take a look at the raw material going into them:

Particle metrics for the first batch of the ultracoarse set.

I specifically choose a more crystalline (aka: blocky) grain for the 300 µm stone. 300 µm is massive – that’s already 0.3 mm, or 0.011″ – about 6 blonde hairs, side by side! By using a more blocky shape, the surface morphology left after the sharpening action is smoother. But because diamond is so unbelievably hard, it still cuts freely. The funny thing is: the markting buzzwords you read about “soft cutting” or “sharp edges” don’t really apply in what we are doing here. Science has long defined what a “sharp” cutting edge looks like in subtractive manufacturing: the rake angle, but also the relative tool sharpness (RTS) play a huge factor. Relative tool sharpness is defined as your uncut chip thickness, divided by your cutting edge radius. This means: if the chip is larger than your cutting edge radius, you get a RTS>1, which is typically meant as “good cutting”, with proper chip formation. At RTS around 1, your specific cutting energy typically increases – more effort is wasted on things like friction, rubbing, elastic and plastic deformations. At RTS much smaller than 1, you might even get into a “burnishing” range, where plastic deformation dominates. Now, the cutting edges we “employ” are not the shape of the grain – instead, you have to look at every edge, every crevice and every nook on the grain as a potential cutting edge. Therefore, the actual shape of the grain does not really change whether it’s soft cutting or hard cutting, but the shape may very well project itself on the blade we try to sharpen. My decision to use a blocky grade for the 300 µm stone thus does not change the way it cuts – it just improves the surface morphology creation.

Let’s take a look at the stones:

The very first set of Dr. Marv’s Scientific Sharpening Stones –

The stones are REALLY green. That is because the diamond inside these stones is greenish in colour, and there’s no fillers or other additives in these stones besides the diamond.

PLACEHOLDER: SEM pictures of broken through stones. Mea culpa, forgot to copy those. Will add these on monday or tuesday.

These stones are designed to re-shape a bevel, help create one or rescue a ruined knife. They really move a lot of material! I’ve decided to expand my testing procedure with a 2nd steel.

The first steel is Boehler M398. I absolutely love M398 – it polishes beautifully, has great edge retention and decent corrosion resistance. It contains quite a high carbide content, most noticeably in the form of Vanadium carbides.

It is the steel I’ve used for all of my blog reviews so far – and has been hardened by my good friend and the heat treat virtuoso Roman Kasé to 65 HRC.

New in future reviews is a more “common” steal with a commercial heat treatment. I’ve decided on NitroV – and bought a couple of Civivi Sendy knifes.

NitroV is basically AEB-L, with 0.1 % Nitrogen added in. This makes it very comparable to one of the most used knife steels, and probably a steel (or very close!) that everyone of my avid readers has lying around, somewhere. I’ve decided on a commercial heat treatment, because I wanted to have an opposing data point to the custom heat treat M398 I typically use.

The steel measured at 59-60 HRC, and chemical composition inside the SEM checks out to NitroV.

In the following, I have sharpened both M398 and the NitroV blades with the new stones. My usual approach to sharpening comes to use here. They got used with oil.

Let’s take a look at the edges from the 300 µm stone:

SEM micrographs of the M398 edge – 300 µm stone. Instrument: Thermo Fischer PhenomXL SEM.

SEM micrographs of the NitroV edge – 300 µm stone. Instrument: Thermo Fischer PhenomXL SEM.

Honestly, when I started work on these stones, I expected these to be massive material hogs. And they are! The blade is just disappearing as you grind along. But the resulting apex is actually already pretty fine, there’s no massiv burr or prow formation, and the surface finish is not too bad. This checks out under the optical microscope, too:

Optical micrographs of the M398 blade (first 3) and the NitroV blade (last picture) after sharpening with the 300 µm stone. Instrument: Leica Emspira

In order to give you some sense of the speed, I’ve compiled some examples for you from my own use:

150 micron stone: 1 min per side to get from beltground finish to 17DPS apex on REX121 (70 HRC)
300 micron stone: 2 min on M398 (65 HRC) to change bevel angle from 17.5° to 17°
300 micron stone: complete rework of the edge of a Civivi Sendy in Nitro V (60 HRC) in < 2 min

Obviously, speed is something very subjective – it depends a lot on the steel used, your pressure and the condition of the stones. To me, this feels like the fastest ever resin stone I’ve used. The 300 µm feels faster than the ATOMA F400, but leaves a better finish. The 150 µm one feels pretty similar in terms of speed, but leaves a much nicer apex for me. Speaking about the 150 µm one, this is the apex created with it:

SEM micrographs of the M398 edge – 150 µm stone. Instrument: Thermo Fischer PhenomXL SEM.

SEM micrographs of the M398 edge – 150 µm stone. Instrument: Thermo Fischer PhenomXL SEM.

Honestly, I’m stumped. This is a fantastic apex – and the surface is looking really good, while this is the “coarsest” stone I’ve had on this blog (with the exception of my 300 µm one!). This is a super nice result. I’ve been playing around with prototype stones in this grit range for quite some while, and they’ve been used to set the angle on most blades for my review for the past couple of months. The stone is super long lasting, super quick and… I absolutely love it.

Optical micrographs of the M398 blade (first 2) and the NitroV blade (picture 3&4) after sharpening with the 150 µm stone. Instrument: Leica Emspira

A word on their use: the stones are really agressive. And just like all non-EP stones, they loose grains. On these stones, with such a large diameter grain, this is more noticeable than on fine stones, as you can immediately spot these diamonds. I think I’ve managed to get a better grain retention going than most stones. Nevertheless, I would advise to either clean the loose abrasive very carefully from the blade – no pressure while wiping it down – or even flushing them off with some water or oil.

This “review” feels a lot like an advertisement to me. Well, maybe because it is one, it is after all my own product. It is one I didn’t really want to make, and I’m very happy a couple of friends pushed me to pursue this. It’s exceptional, and the surprise in how good the performance is makes this even better. Thank you, dear reader for following my journey.

The stones will be available in a very limited run in my webshop, beginning of February 2026.

This is part of a series of blog posts – looking into the appearance and composition of commercially available sharpening stones. If you are interested in the previous episodes, check out the archive for them.

If you have some suggestion on what I should look at next, or want to share your super secret DIY stones, I could be persuaded to open the bag of analytical devices… hit me up on Instagram under @marvgro for that.

Disclaimer: I’m not for sale. Every review you see on this blog is bought with my own money. I have no affiliation to any manufacturer.

Review

Today’s sharpening stone is the smaller grit brother of the last instalement in this series – the PDTools Vitrified Diamond in grain size 40/28 µm. Typically, this grit size gives nice, toothy edges – ideal for cooking knives, especially if reworked with a very fine diamond to further refine the apex. With so many parts of this “brief study”, one notices a pattern in my stone purchases: it’s either very coarse, a medium grained stone such as this one here, or a very fine one. I do this, because I am interested in how the manufacturers manage to deal with challenges such as grain retention, cost vs concentration considerations, but also agglomeration. I do not test full series of stones, because frankly: I purchase these stones with my own money; most stones see my blog review, and then are spending the rest of their life in a drawer and don’t get used.

Onwards to the review of this vitrified diamond stone!

Optical micrographs of the PDT Vitrified Diamond 40/28 stone. Instrument: Leica Emspira

The optical micrographs are already very interesting. There’s definitely some much larger than expected grains in this stone – but also, quite a bit of diamond can be made out. This will be one interesting stone!

SEM micrographs of the PDT Vitrified Diamond 40/28 stone. Instrument: Zeiss GeminiSEM 560.

The view from the optical microscope is once again confirmed – between the diamond particles, which clump together a bit, are much larger grains visible. To me, the vitrified bond looks very similar to the one in the 100 micrometre stone review – not super high vitrification, with lots of filler particles. Seems like the same size particles was chosen for this finer stone.

Let’s look at the chemical composition! For this we are going to use an advanced SEM technique called EDS. If you want to know more about this, I’ve written extensively about SEM microanalysis here on this blog.

EDS analysis of the stone. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

The matrix is a standard vitrified bond, consisting of different oxides. The diamond is not as nicely distributed as in the coarser brother. It is quite difficult to mix powders well – here, we can find spots that are nearly empty of diamond, and others where it nearly clumps together. I find the addition of titanium once again quite curious – this probably gives the bond some tensile strength when used in CNC applications? Larger, hard oxide particles can also be easily spotted – these are several times larger than the diamond inside the stone.

In order to evaluate the sharpening performance and material removal mode of this stone, a blade was sharpened with it. I am using a standardised testing procedure, read about it here. Nevertheless, it’s 65 HRC M398, and sharpened to 17 DPS with resin bond diamond stones down to 10 µm. Afterwards, the tested stone is used, first in a back and forth movement until the surface becomes homogenous, and then alternating strokes (5-5-3-2) on each side, for a total of 20 strokes towards the apex per side. No pressure is applied but the weight of the apparatus.

The edge is then analysed in the electron microscope for breakouts and morphological appearance.

SEM micrographs of the edge finished with the PDT Vitrified Diamond 40/28 stone. Instrument: Thermo Fischer PhenomXL SEM.

Uff. I’m not really sure what to say here. The bevel shows some deeper scratches, also some scratches are not very straight – this is typically the sign of a particle coming loose and rolling around. The real issue I find in the apex – just like with the coarse vitrified stone, we do not have a clearly defined, cut apex, and also not really a formed burr. Instead, the whole apex is pushed over – plastic deformation instead of material removal. This structure feels to the thumb like a burr – but actually is not, and will make subsequent, finer grit sharpening steps more difficult, as they will have to abrade more material. I’m once again a bit disappointed in this one – vitrified stones have a near-mythical reputation, and this one for sure lives up to the hype created by paid youtubers – something that feels like a burr is formed in very few strokes. At high magnifications, this turns out to not be a burr, but bending of the apex and some scratched bevel. This is once again shown in the optical pictures, which also reveal the massive amount of scratches on the blade:

Optical micrographs of the blade sharpened with the PDT Vitrified diamond stone. Instrument: Leica Emspira