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.