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 brand new, just released EdgePro Diamond Max! Rumors of these stones coming have been floating around for a couple of months already, and I have been very much itching to get my hands on one of these. The Edge Pro Matrix stones (also sold by the producer, CGSW) are considered some of the best sharpening stones on the market – rightfully so! I had their 5 µm Matrix stone on the blog quite some while ago. It gives a fantastic edge, polishes the bevel finely and I only had two major issues with it: it is probably the most expensive sharpening stone on the market by amount of abrasive you are buying, and the low concentration made that stone very slow.
When they got launched in a silent reveal at the end of April, I didn’t hesitate a single second, eager to try it out and order 3 stones to Germany. Something I want to highlight here: the contact with the owner of EdgePro, Cody, was superb – I had a question about tracking and got a super polite, helpful email back. This, dear readers is why I advocate buying from small manufacturers, and ideally directly from them!
Just two days ago, Cody uploaded a video on his youtube channel explaining about these stones. The new DiamondMax stones seem to adress the issue of speed, albeit this comes hand in hand with a major price bump (roughly 20$ increase on the stones, bringing this 160 µm / 80 grit stone to 107$ before taxes / import duties if you buy outside the US). The abrasive layer is still very thin. According to the manufacturer, these stones now contain between 1.8 and 2.5x more diamond, depending on their grit!
Let’s take a look under the optical microscope!
Optical micrographs of the stone. Instrument: Marvscope
I absolutely love coarse sharpening stones under an optical microscope! There, one really can make out the grain and grain concentration. The stone has a high concentration of quite blocky diamonds, a natural appearance at that grit size. The grains tend to clump together in groups of 3-6 grains. In between the grains, we can see the white resin layer. Some much smaller, blue-blackish particles can be made out.
I do not own a comparable size Diamond Matrix stone (hold your curiosity until I get around to the 5 µm review, there I can do a direct comparison!). The producer of the stones seems to still be CGSW, and he is very curious about my sharpening stones. In this thread on the bladeforums, David writes:
“…Diamond Max series that didn’t go anywhere. I made 2 sets of Matrix stones with the most diamond in them that I could a few years ago. One for EP and one for me. They do cut a little faster but at the expense of wearing much faster. Even if they didn’t cost more they would be a lower value than the current Matrix stones so they didn’t make it to production.” Quote from user “Diemaker” on the bladeforums, accessed on Sunday, 24th of May 2026.
Uff. Okay. That’s a hardcore statement to make about a product that is yet to launch.
Let’s take a closer look in the SEM:
SEM micrographs of the EdgePro DiamondMax 160 µm stone. Instrument: Zeiss GeminiSEM 560.
Under the SEM, the blocky nature of the grains is further confirmed. Size seems to peak at 160µm, with the majority of grains slightly smaller (100-140 µm). It is normal that diamond powder is not a single size, a gaussian distribution is always expected. What I find very curious is the high amount of “voids”, where clear imprints of grains have sat before. This is the stone before use, and already a massive loss of grains can be made out. Zooming in on one grain, we can see that although the resin is confining the grain above it’s main diameter, it is already loose and there is some gap between the resin and the grain.
Grain adhesion is the major issue in most resin stones, and it becomes more dominant the larger the grain becomes.
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 EdgePro DiamondMax 160 µm stone. Instrument: Oxford Ultim Max ∞ 40mm2 EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.
The stone shows the typical, very pure composition we already saw in the Matrix stones: There’s diamond in there, and an organic binder, with not much else. The black-blue grain we made out in the optical micrograph shows as an oxide-abrasive grain, mostly peaking on the Mg-Si-O channel. I would guess that this is some abrasive debris from their flattening process. It will probably disappear after a few sharpening cycles, leaving a pure stone behind.
The impression that the diamond grain seems to clump together a bit is further confirmed in this image – we can see small nests of diamond.
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, edge trailing 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. Moreover, the same approach is repeated with a blade in NitroV at 59-60 HRC.
The edge is then analysed in the electron microscope for breakouts and morphological appearance.
Something that became immediately apparent when using the DiamondMax stone, and was already suspected from the SEM pictures: this stone looses a lot of grains! Let me show you what I mean:
Microscopic views of the loose grain /swarfs after 20 and 100 strokes with the stone. Instrument: 100x Macro Loupe on iphone 17 Pro Max
Already on the first stroke (edge trailing), one could feel how grains would jump out. I counted to 20 strokes, and then did a picture of the bevel with my phone. You can see a frankly absurd amount of diamond – and very little swarf. Over the next 80 strokes, the stone picked up some speed, producing a lot of swarf, but also loosening even more grains. I cleaned it off, applied new lubricant and the same thing happened. Initial, hard grain loss, followed by an increase in material removal rate once there’s a certain “slush” going on. I’m a bit stumped by this wear rate – and frankly, if you sharpen any expensive knife, you do not want this massive amount of loose grains potentially scratching the surface.
Let’s take a closer look at the result, and start with the harder steel – the M398 blade:
SEM micrographs of the M398 edge finished with the EdgePro DiamondMax 160 µm stone. Instrument: Zeiss GeminiSEM 560
We can see quite the ragged edge. The bevel shows clear signs of the rolling grain – deep scratches in the middle of the bevel, stopping and starting randomly.
The overall appearance is diffuse and sligthly chaotic – this is because the rolling, loose grain can jump around, but also move sideways and not only in the direction of the sharpening stroke.
Optical micrograph of the M398 bevel. Instrument: Marvscope
Which is further visible in the white light interferometer measurements of the bevel: a diffuse, marred surface:
3D surface height map of the M398 Bevel. Instrument: Zygo NewView 9000, Objective Lens: 20X. Metrological filter chain: LS-Plane to orient data, cutoff 0.1/99.9 percent to remove outliers.
The surface roughness is pretty rough, too:
| Sa | 0.3708 | µm |
| Sq | 0.5017 | µm |
| Ssk | -0.7942 | – |
| Sku | 5.777 | – |
ISO 25178 surface roughness parameters. S-Filter: 2.5 µm (gaussian), L Filter: 0.08 mm (gaussian). No F operation besides LSQ leveling.
Overall, I’m quite disappointed. Let’s see whether the stone performs nicer in a softer, easier steel, and take a look at the NitroV edge:
SEM micrographs of the NitroV edge. Instrument: Zeiss GeminiSEM 560
In addition to the very rough, broken up surface, we can also detect some splintered pieces of diamond that have embedded themselves into the bevel.
Optical micrograph of the NitroV bevel. Instrument: Marvscope
The surface looks a bit more irregular, with a massive amount of sideways or circular scratchmarks, caused by the grain rolling around freely in the abrasive/debris slush created.
3D surface height map of the NitroV Bevel. Instrument: Zygo NewView 9000, Objective Lens: 20X. Metrological filter chain: LS-Plane to orient data, cutoff 0.1/99.9 percent to remove outliers.
The surface roughness deteriorates even more:
| Sa | 0.4595 | µm |
| Sq | 0.6215 | µm |
| Ssk | -0.8645 | – |
| Sku | 5.439 | – |
ISO 25178 surface roughness parameters. S-Filter: 2.5 µm (gaussian), L Filter: 0.08 mm (gaussian). No F operation besides LSQ leveling.
Now, you might think at this point: why am I so disappointed? It’s a coarse stone, meant for quick material removal, and some grain shedding is expected, especially on resin stones.
The problem is: it’s not. It can be done differently, it can be done better.
Let me explain, and I’ll do so by something I do very rarely – a direct comparison. You see, I also make a resin stone with near identical grain size denomination, the Dr. Marv Scientific Sharpening stone in 150 µm. Let me pull you up an optical picture, side by side to the EdgePro Diamond Max in 160 µm:
Identical magnification shots of (first picture) the Diamond Max 160 µm and (second picture) the Dr. Marv 150 µm stones.
I will let you draw conclusions about the concentration yourself. Let us compare the results – this is the exact same M398 blade, sharpened with my stone:
SEM micrographs of the M398 edge finished with Dr. Marv’s 150 µm resin stone. Instrument: Zeiss GeminiSEM 560
The view of debris after 20 and 100 strokes:
View of the swarf after 20 and 100 strokes, sharpened with Dr. Marv 150 µm resin stone.
Optical micrograph of the bevel:
Optical micrograph of the M398 bevel. Instrument: Marvscope
And last but not least, the WLI results:
3D surface height map of the M398 Bevel. Instrument: Zygo NewView 9000, Objective Lens: 20X. Metrological filter chain: LS-Plane to orient data, cutoff 0.1/99.9 percent to remove outliers.
With the surface roughness values being about 2.5x lower than on the EdgePro stone:
| Sa | 0.1481 | µm |
| Sq | 0.1981 | µm |
| Ssk | -1.003 | – |
| Sku | 4.930 | – |
ISO 25178 surface roughness parameters. S-Filter: 2.5 µm (gaussian), L Filter: 0.08 mm (gaussian). No F operation besides LSQ leveling.
I typically end my reviews with a conclusion. I think none is needed here, but for completeness sake I’ll do one:
The DiamondMax 80 grit stone seems to have a high diamond concentration. It is definitively not the maximum possible. Some agglomeration is apparent in micrographs and chemical analysis. Grain retention is nearly non existent on the stone, with large amounts of wear and free-rolling grain induced results on the blade. The stone is probably the most expensive diamond stone on the market if one takes the very thin arbasive layer into account. Results are matching these findings – marred, rough bevels and a ragged, wavy apex. The initial quote I pulled from David (CGSW) on the Bladeforums becomes very true:
“Even if they didn’t cost more they would be a lower value than the current Matrix stones “
And they even bumped the price.
