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 something REALLY special. They are resin based, CBN sharpening stones from St. Petersburg – made from the special “elbor” type of CBN. Historically, this CBN was characterised by a lower content of CBN, with impurities of hexagonal boron nitride and MgO. Nevertheless, these impurities resulted in blocky shapes with defects such as cavities and inclusions. These increased the surface area, increasing bonding strength to resin, but also self sharpening properties. This is advantageous for their use where low forces and low heat is prevalent – for example, hand sharpening of knives! If you are more interested in reading about elbor CBN – check out this paper:

Volov, V.N., Garshin, A.P. Comparative Indices of Different Grades of Cubic Boron Nitride Abrasive Powders. Refract Ind Ceram 61, 441–450 (2020). https://doi.org/10.1007/s11148-020-00500-5

The CBN stones I have lying here are also true historical ones – as far as I am aware, they were manufactured in 2025, but the CBN powder is from old soviet productions. Truly something special! I was gifted these sharpening stones by Nickolay – thank you very much for your continued support! At this point in time, the manufacturer of these stones does not have a homepage. I will update this review if one becomes available.

The stones are the typical 1×6″, guided sharpening stones style. They came in a nice, printed box and are laser marked to their respective sizes:

Photographs of the sharpening stones from St. Petersburg.

If, like me, you are unable to read Cyrillic font, don’t worry, I got you: The laser marking reads “ELBOR”, the grain size and then in a 2nd line the description “super soft bond”.

Under the optical microscope, more of the bond details are revealed:

Optical micrographs of the 20 µm stone. Instrument: Leica Emspira

We can see a white bond, interspersed with very dark and slightly lighter grey particles. Zooming in, a few bubble spots can be made out, but also that the dark grey spots consist out of agglomerated grains.

Optical micrographs of the 10 µm stone. Instrument: Leica Emspira

This is also visible on the 10 µm stone. I’d guess that this is the same bond, with just a smaller abrasive. The grey spots are smaller, and grains are harder to make out in these.

Optical micrographs of the 5 µm stone. Instrument: Leica Emspira

With decreasing abrasive size, the number of dark grey agglomerated spots increases. This could point towards some trouble with mixing?

Let’s take a closer look under the SEM, to identify what the large black agglomerates are, but also what the bond consists of and looks like:

SEM micrographs of the 20 µm stone. Instrument: Zeiss GeminiSEM 560.

This stone has some topography to it! We can see a number of (probably) CBN particles, fitting the size stated on the stone. The concentration is not super high, but a lot of grains are visible. Interspersed are some voids / bubbles, but also some massively larger particles. The bond itself seems to be very fine and regular, with some tiny filler particle in it. The CBN grains themselves definitely look like Elbor grade -they are irregular shaped, having a sharp appearance.

SEM micrographs of the 10 µm stone. Instrument: Zeiss GeminiSEM 560.

The 10 micron stone looks very similar – but also, the larger filler particles in this stone are easier to make out! The concentration is similar to the 20 µm stone.

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

For the 5 µm stone, we have a lot of pictures to dig thorugh. In the overview, we can see that this stone also has large filler particles that most likely aren’t CBN, as they are flat ground. But there’s also one of the spots of agglomeration we were able to make out in the optical microscope here! Zooming in on that one, we can see that it contains a lot of loosely held abrasive particles. This is a clear case of agglomeration. It is only natural that on the finest stone, this is the most visible, as it’s more difficult to properly mix fine powders.

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

We can see the distribution of the CBN particles (N, B) all over the FOV. Moreover, we can see that the resin bond is heavily reinforced through ceramic filler particles. Most noteably, oxide ceramics build from Al, Mg and Ca.

EDS analysis of the stone 10 µm stone. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

The same bond is visible on the 10 micrometre stone. Here, we can also make out one of the large “super particles” we spied in the SEM overview. It looks like alumina oxide.

EDS analysis of the stone 5 µm stone. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

This continues on the 5 µm stone – same bond, ceramic reinforced resin bond. But here, we can also make out the agglomerated part on the far right corner of the FOV. Let’s focus a bit more on this one:

The particle agglomeration is mostly CBN – the fine particles clumped together, and during dressing they formed a small void, as only the ones with contact to the resin bond remained behind. The red particles around it are CaO.

Now from morphology and elemental analysis, I did not expect a good performance from this stone. Previously in this blog, all stones that showed massive amounts of agglomeration and larger fill particles behaved badly during the sharpening action. But this is why I always sharpen a bit with the stones and prepare a blade edge!

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. These stones were tested in NitroV at 60 HRC. I’ll supply M398 pictures at a later date, but our small SEM is currently broken – bear with me for a couple of weeks! For now, enjoy HD pictures form the big SEM.

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

SEM micrographs of the edge finished with the 20 µm stone in NitroV. Instrument: Thermo Fischer PhenomXL SEM.

The 20 µm stone leaves a very regular appearance on bevel. Some deeper scratches are visible from time to time. The cutting edge has a large, foil type burr that is quite ragged. This slices easily into a piece of paper!

SEM micrographs of the edge finished with the 10 µm stone in NitroV. Instrument: Thermo Fischer PhenomXL SEM.

The 10 µm stone further refines this edge. The overall surface morphology improves, becoming smoother, but some larger scratches come through, still. This is most likely the super large aluminium oxide particles we have seen in the SEM. The cutting edge shows less burr, and some ragged parts remain. The blade was quite sharp at this point!

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

Lastly, the 5 µm stone further refined the surface. We have a micrometric foil type burr remaining, some deeper scratches and clear prow/burr formation visible in the surface morphology. The chamfer itself is shiny, but not glossy. The apex is not super fine – I think this stone exhibited quite a bit of pressure from the ceramic fillers, breaking off the burr and leaving a wider-than-necessary apex. It is in my personal opinion the weakest stone out of this set – I’d guess that going to 10 µm and then stropping would leave you with the best and well formed edge.

So, a final and very subjective conclusion: the stones themselves are wonderful. Nice feedback, quick acting, low tendency to load and give a sharp edge with a very regular, homogeneous grinding pattern. They do burnish a bit and cut less clean than a pure CBN or diamond stone does, which leaves one with a larger burr than one would expect. The sometimes very large oxide ceramic particles, but also heavy agglomeration on the smaller sized probably reduce the performance. Nevertheless, this stone set is high performance and because of the historical CBN powder something very special – they will remain in my collection as a cherished set and will be used from time to time.

I have zero ideas how one would go about buying these – my understand is they are made to order and probably impossible to get in the western world. If you do have a chance to snag a set – I’d go for it.