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

Remember, when you started to sharpen? And you got introduced to stropping? And someone gave you an old leather belt, and some green goo you rubbed on it? Mabye later you decided to put some fancy diamond emulsion on the same piece of leather. It got your edges sharp, but looking back you wonder why you thought mixing to abrasives was a good idea. This review made me think of this specific situation. I’m happy to say I never did the above mentioned, but hold tight, for:

Today’s sharpening stone is another KDTU, this time a very fine one – the hybrid diamond in 2/1 µm declaration.

Optical micrographs of the KDTU Hybrid Diamond 2/1 µm stone. Instrument: Leica Emspira

It’s a very green stone. Under the microscope, it once again shows large spots and particle-regions that are a different colour. This time, it differs between a smooth green for the majority of the stone, some darker green spots and some lighter spots that appear nearly white. I’d like to quote from my previous review of these stones what the manufacturer states on his homepage about the colours:

“Hybrid bond whetstones may have some multi-colored streaks on the surface of the working layer, like on natural stones. They may differ in shades. This depends on the size of the abrasive grain in the whetstone or be lighter or darker even on the same grains. The quality of sharpening whetstones does not depend on the shades and color of the working surface.” – KDTU Homepage on bond colour, accessed on Sunday, 1st of February 2026

With the 14/10 µm sized stone, the “multi-colored streaks” turned out to be massive amounts of agglomeration. But this time, the stone is very green. I fear for the worst… so onwards, to the electron microscope!

SEM micrographs of the KDTU hybrid 2/1 µm diamond stone. Instrument: Zeiss GeminiSEM 560.

We can see a very compact, glassy looking surface. Moreover, already in the first shot, we can see one large area where agglomeration seems to reign supreme – it is half a bubble, where probably unmixed abrasive already fell out, and left this dimple of unmixed particles. It is very similar to the coarser brother of this stone, but the surface is overall much smoother and denser. This will be an exciting stone for the elemental 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.

When the first signal from the EDS sensor registered, and I saw a lot of chromium, I immediately thought: that can’t be real. But by collecting more signal, it become clear – the green colour in this stone is from massive amounts of chromium oxide. Chromium oxide is the cheap, green compound you get on beginners kits for stropping. It’s a decent choice to remove a burr on low tech steels. At the corners of the image map, we can see larger concentrations of diamond – also agglomerated. Zooming out further highlights this:

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

The whole bond of the stone seems to contain a lot of chromium oxide. In between is some diamond, but the spots one can even make out with the naked eye are unmixed, agglomerated sections of diamond (red channel, carbon) or spots of “pure” chromium oxide (green channel). Pretty wild! Let’s see how this performs:

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. For this fine stone, I continued to the 2.5 µm Dr. Marv stone, leaving a near perfect mirror and a hair whittling edge. 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 stone. Instrument: Thermo Fischer PhenomXL SEM.

We get what can be expected – some large scratches, where these agglomerated particles bunched together and left a deeper scratch. Moreover, the apex, is slightly rounder than I would expect of such a fine diamond stone. This looks pretty similar to “overstropped” edges – because even in M398, the chromium oxide is pushing and burnishing and deforming the edge. The blade got noticeably scratched and duller during the use of the stone.

Optical micrographs of the KDTU 2/1 µm stone. Instrument: Leica EMSPIRA.

Overall, I’m really disappointed in this stone. The decision to use quite a lot of chromium oxide in a bond is curious. I could imagine, that the use a stropping compound appealed to the manufacturer in such a fine grained stone. It reminds me of the scenario I started this review with. But mixing is frankly atrocious, and the final performance of this stone falls very much short. The stones from KDTU got hyped a lot on youtube in the past few months, which made me buy a couple. All of them work, none of them work exceptionally well, which can be contributed to agglomeration, curious bond decisions and bad mixing. This makes me question how much influence or paid content there is.

I think there are much better choices out there – and thus can only conclude: Save your money on this ultrafine stone and grab a simple leather strop with some quality diamond emulsion. You’ll get better results by far. Alternatively, if you detest stropping as much as I do, there are some fine stones out there, that are well mixed, high concentration and perform insane.

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 review is for my own product, so therefore it could be considered an advertisement in certain countries. You are hereby warned.

Review

Today’s sharpening stone is the 2nd member of my “experimental series”. Reading my blog, you might have discovered: This young *coughs* Doctor loves diamonds. They are sparkly and shiny and my favourite abrasive. Nevertheless, I’m a naturally curious person. Many abrasives are used to shape metal – some more on the soft side, some are added as filler to the stones we all use.

In my “experimental series”, I explore the concept of different abrasives in their pure form. You could consider my Scientific Sharpening Stones the “diamond version”, and last year I published (and released a very limited edition!) of the experimental series in CBN. They are called “experimental series” for a reason – I do not think that these stones will outperform my Scientific Sharpening Stones. I actually would be surprised if they have a higher performance. But I am unable to test them in every condition, every steel and also – sharpening is a very subjective thing. Maybe some people will love the edge produced by this. I think some of my avid readers might be interested in trying this out – and become the scientist themselves through their experiment! 

Today I’m introducing my SiC experimental series – and I can already tell you: surprisingly enough, I absolutely love them.

SiC is an interesting abrasive – I’ve written about it before in my “abrasive snippets” section. It’s quite hard at around 2800 HV (CBN: 4500-5000 HV, Diamond: 10000 HV, most steels: 800-1000 HV), very inert, temperature stable and wonderfully bonds to resin.

SiC exists in many different structures, which are called polytypes. For these stones, I’ve used the harder type, 6H – SiC, which is black. It’s more brittle than it’s green cousin.

The stones I made are 30, 15 and 5 µm size, and have a wonderful, black-greyish colour:

A picture of the very first “SiC Experimental Series” Stones.

Let’s take a look under the SEM – stone by stone. For this, I’ve broken a stone in half to enable us to look at the cross section:

SEM micrographs of the 30 µm stone. Instrument: Zeiss GeminiSEM 560.

Something that immediately stands out is not only the high concentration – but also the success in distributing the grain: SiC often clumps together in commercial sharpening stones, but here we have a fantastic mix and wonderful distribution over the whole Image!

To test it’s performance,, a blade was sharpened with my typical approach, read about it here. As this is a relatively soft abrasive, I’ve sharpened a blade in NitroV at around 60 HRC. While the stones do remove material (I’ve tried it with good success in Magnacut and M390, and with very little success but some material removal in M398 at 65 HRC), this is an abrasive best suited for old fashioned, lower carbide content steels.

SEM micrographs of the edge finished with the 30 µm SiC stone. Instrument: Thermo Fischer PhenomXL SEM.

The 30 µm stone is a beast. It actually abrades material quite quickly, after just a few strokes there’s a lot of chips swimming in the thin coating of oil on the blade. The apex itself looks pretty toothy, but also quite refined. I’m really surprised here – but this is actually very decent, considering it’s no superabrasive!

Next, let’s take a look at the 15 µm stone:

SEM micrographs of the 15 µm stone. Instrument: Zeiss GeminiSEM 560.

Once again, a wonderful distribution and quite high concentration. I’ve continued sharpening the NitroV – but want to share picture from the sharpening progression here:

Swarf and chips on the blade after sharpening for about 30 seconds with the 15 µm stone!

Do you see the amount of swarf, swimming in the oil? These stones definitely cut! Let’s take a look at the apex:

SEM micrographs of the edge finished with the 15 µm SiC stone. Instrument: Thermo Fischer PhenomXL SEM.

This was already a really sharp blade. Sharp enough, that it easily sliced into a tissue. We see both a refinement on the surface, but also the apex.

Last but not least, we need to take a look at the 5 µm stone:

SEM micrographs of the 5 µm stone. Instrument: Zeiss GeminiSEM 560.

This stone seems to have an even higher concentration – but as I do my stones in “weight” and not some obscure, hard to verify volume concentration, it’s exactly as much abrasive as on the previous stones. But: With the smaller grain, this means a much higher count of potential cutting edges. The blade was then further sharpened with the stone:

SEM micrographs of the edge finished with the 5 µm SiC stone. Instrument: Thermo Fischer PhenomXL SEM.

This is actually a pretty decent apex. It’s thin, pretty regular at normal magnifications. Only at 1000x, the toothy and waviness really becomes visible. Some part of the edge has been slightly folded over in a foil type burr, which wasn’t really visible with the 15 µm stone. The surface finish of the bevel is further refined, but still shows a plethora of scratches.

The stones themselves are pleasant to use. They have a bit more feedback, which probably stems from all the sharp, pointy SiC grains. If you’ve read my review about my experimental series in CBN, you probably got the message that I was a bit disappointed – they were good stones, but fell shy of my diamond stones. I’m not saying that these SiC stones perform better. They definitely don’t. But they are fun to use, and they give a blistering edge – and most of all, the surface finish left on the bevel is very special. Instead of a perfect polish, the surface is marred by thousands of fine scratches, giving a shiny, matte appearance, that changes colour in thousands of rainbows when you twist and tilt it in the light:

The appearance of the edge under a Leica DMS 300 microscope. The light source is moved back and forth along the apex.

Honestly, this is not my highest performing stone. The CBN stones are objectively better at slicing through high tech steels. But: they are fun to use, if the steel they are used on is a good fit. They have a pleasant feedback. The bevel created is wonderful – it’s exciting, ever changing and has some character that a perfect mirror never could have. I honestly love these stones. My good friend Roman Kasé, when I told him about this experience didn’t stop laughing and told me “welcome to the oldschool feedback gang!”. Not sure I am there yet, but these stones WILL get used again and again.

Dr. Marv’s Experimental Series in SiC is available now, in a very limited, individually numbered edition of 10 sets.

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 another KDTU stone. We’ve had their hybrid CBN on the blog before, albeit in a much coarser grit. This time, it’s the Hybrid Diamond version, in the relatively fine 14/10 µm declaration. Let’s take a closer look:

Optical micrographs of the stone. Instrument: Leica Emspira

Immediately visible is – the stone is “spotted” all over. Quite a few smaller agglomerates, but also larger sections were the stone is not homogeneous. At the time of this review, the manufacturer writes on their homepage:

“Hybrid bond whetstones may have some multi-colored streaks on the surface of the working layer, like on natural stones. They may differ in shades. This depends on the size of the abrasive grain in the whetstone or be lighter or darker even on the same grains. The quality of sharpening whetstones does not depend on the shades and color of the working surface.” – KDTU Homepage on bond colour, accessed on Sunday, 1st of February 2026

I’m a bit skeptical. In my experience, bad mixing leads to sections that are inhomogeneous, and those agglomerations act like particles that are much larger, giving deeper and wider scratches. Fortunately, we can check whether it’s just colour or actually something else. Into the SEM!

SEM micrographs of the stone. Instrument: Zeiss GeminiSEM 560.

In the SEM, we can see a standard mixed resin bond, but also large pockets, that seem to consist only of hard, similar sized grains. These pockets likely were “full” once, but lost the majority of their filling during dressing or cleaning of the stones. Inside the bubble shaped recession, one can make out many similar sized grains. I’m curious what these particles consist out of.

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 EDS detector shows that the bubble particles are mostly silicon – some oxygen is visible as well. I would guess that it is hard particles of SiO2? Maybe some smaller SiC grains? The analysis is not fully concise here, it’s difficult to get the xray signal out of a pocket. A second location shows a lot of SiO2 particles, with a decent diamond concentration that tends to clump together. Overall, this stone is pretty badly mixed. Let’s check whether this impacts the final result!

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 KDTU stone. Instrument: Thermo Fischer PhenomXL SEM.

I’m not super happy, but also not super disappointed with this edge. The apex itself is okay-ish formed, it shows some deeper grooves than I would expect at 14/10 µm, which likely stems from the larger particles. The surface of the bevel meanwhile looks really rough. Lot’s of small prows, burrs and smearing is visible. This typically shows on M398 when the stone abrasive is too soft – for example, natural stones and SiC / Al2O3 based stones show this effect a lot.

In the optical microscope, this is confirmed: the surface is marred and slightly dull.

Microscope pictures of the KDTU 14/10 diamond hybrid stone. Instrument: Leica EMSPIRA

The stone itself is pleasant to use, with quite a bit of feedback. The edge got decently sharp, but nothing groundbreaking here. In my opinion, this is a mid-level stone that suffers mostly from the bad mixing, inhomogenous makeup and massive amounts of filler abrasives.