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
In the last part of this series, we took a look at the brand new DiamondMax 80 Grit (160 µm) from Edge Pro. The review turned out to be quite the disappointment – the stone was suffering from heavy grain loss. Now, while it definitely is possible to make a resin stone at that grain size that works nicely, the main application of resin bond sharpening stones is the finishing – this is after all something where they shine. Our next look at this new series is the exact opposite end of the spectrum – the Diamond Max 4000 grit (5 µm). I’ve had a Diamond Matrix stone in a previous review.
I’d advise you to check out my review on the 80 grit first:
The manufacturer of the stones, David from CGSW has meanwhile commented under that blog post and given a more detailed insight into the increased diamond ratios. As this is very interesting, I’d like to quote him here:
“To be clear, all Matrix stones have had more than 50% diamond to resin content by weight in them, and I have made every single one of them, so I can say this with confidence. The 80 grit Max stone has a 2.5 times higher concentration of diamonds, the 250 2.2 times higher, the 450 through 1700 have 2 times higher, and the 4000 1.8 times higher concentration of diamonds. For perspective, if they started out at 50/50, then 67/33 is double the concentration. I put the maximum amount of diamonds in the Max stones as is feasible for this resin. If I put more diamonds in this resin I run into processing problems.” – David from CGSW, commenting on my blog (Part 62) on the 24th of May 2026
This is clarifying a point, as it doesn’t mean these contain twice the amount of diamond, it just means the ratio has shifted – 67% by weight is after all just 35% more diamond.
The original Diamond Matrix in 5 µm size is one of the stones I consider very, very good. My major issues back then was the slow speed – let’s take a look at the new DiamondMax in 5 µm!
As always, we will start under the optical microscope:
Optical micrographs of the Edge Pro Diamond Max 5 µm stone. Instrument: Marvscope
The stone has a very homogeneous, regular appearance. The diamond can barely be made out at this magnification – just like I would expect of a 5 micron stone!
Let’s take a closer look in the SEM:
SEM micrographs of the Edge Pro Diamond Max 5 µm stone. Instrument: Zeiss GeminiSEM 560.
We can see that this really is a high concentration stone! there’s diamond just about everywhere on the stone. No foreign particles jump out immediately. Unfortunately, just like with the 160 µm stone, some voids can be seen, and the remaining particles don’t show perfect, solid embedding either.
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 Edge Pro Diamond Max 5 µm stone. Instrument: Oxford Ultim Max ∞ 40mm2 EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.
The EDS definitely confirms the view – this is a LOADED stone. Lot’s of diamond. The distribution is good, but not perfect. Some foreign particles can be made out – those are ceramic particles from the manufacturers dressing.
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.
Let’s start with the harder steel – the M398 blade:
SEM micrographs of the M398 edge finished with the Edge Pro Diamond Max 5 µm stone. Instrument: Zeiss GeminiSEM 560
We get a good result here. The bevel is polished, shiny and the carbides are easily identified – typically a sure sign for higher polishing abilities! The apex is smooth, with very little damages visible. At higher magnifications (1kx, 5kx), once can see some scratches that very likely are from rolling, free grain. These are characterised by their appearance in the middle of the bevel – whereas embedded particles in the stone typically show up as scratches that go along the full length of the stone.
Optical micrograph of the M398 bevel. Instrument: Marvscope
Let us take a look at the surface height map:
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 parameters as follows:
| Sa | 0.00713 | µm |
| Sq | 0.009512 | µm |
| Ssk | -0.8962 | – |
| Sku | 5.805 | – |
ISO 25178 surface roughness parameters. S-Filter: 2.5 µm (gaussian), L Filter: 0.08 mm (gaussian). No F operation besides LSQ leveling.
This is a very respectable result! A nanometric surface roughness, especially in the single digit range is a finely polished, mirror like surface.
Let’s take a look at the NitroV edge:
SEM micrographs of the NitroV edge. Instrument: Zeiss GeminiSEM 560
The DiamondMax had a bit more issues on this steel – something that is often seen on soft diamond stones when used on softer, less high tech steels. Nevertheless, we get a fine apex, and a relatively smooth surface. Near the apex, more damage from rolling grain can be seen. The bevel on this test blade is a bit wider than on my M398, I’d guess that this allowed for more swarf to buildup.
Optical micrograph of the NitroV bevel. Instrument: Marvscope
Overall, this is a nicely working stone. Brand new, the stone felt very aggresive, but after the first “familiarising blade”, speed went down. On the tested blades, the stone was quick in the beginning, but got noticeably slower as swarf and loading build up.
Comparison with the Diamond Matrix stone
Now, let’s compare this to the proven and excellent Matrix 4000 – after all, this is the main question here: is it worth it to upgrade?
Let’s take a look at identical condition microscopy pictures of the Diamond Matrix stone:
SEM micrographs of the Edge Pro Diamond Matrix 5 µm stone. Instrument: Zeiss GeminiSEM 560.
We can immediately make out a much lower diamond concentration.
This is further confirmed in the EDS analysis:
EDS analysis of the Edge Pro Diamond Max 5 µm stone. Instrument: Oxford Ultim Max ∞ 40mm2 EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.
The view in the SEM is much more homogeneous than from the DiamondMax, with fewer “rolling grain” artifacts visible:
Edge quality looks pretty much identical, but the surface morphology is more homogeneous.
This is further reflected in the 3D height map:
The roughness is pretty much identical to the DiamondMax stone – I’d say there is no significant difference:
| Sa | 0.008447 | µm |
| Sq | 0.01094 | µm |
| Ssk | -0.4306 | – |
| Sku | 4.170 | – |
ISO 25178 surface roughness parameters. S-Filter: 2.5 µm (gaussian), L Filter: 0.25 mm (gaussian). No F operation besides LSQ leveling.
Please check out my original review of the 4000 stone here:
So, let us compare the results, side by side:
Comparison between the new DiamondMax 5 µm (left side) and the “old” Diamond Matrix 5 µm (right side).
The new DiamondMax stone definitely contains significantly more diamond. Moreover, it is build on the same “principles”, meaning it’s a very pure resin stone. According to the manufacturer, the resin hasn’t changed at all. The result is comparable in nature – the measured surface roughness is within the variance expected. The apex is comparable in quality. The DiamondMax feels ever so slightly faster, but I wouldn’t call it a significant difference. I feel like it looses more grains – this would go hand in hand with the manufacturers statement that it wears quicker, but also what we can identify as a slightly more irregular scratch pattern on a bevel due to rolling grains. Overall, I would say this is a minimal step forward in terms of speed, but it looses some of it’s quality by this. If I was you, I’d stick with the old Matrix 4000, this upgrade doesn’t look like it’s worth it.
Just like in the last review, I want to draw the comparison with what a pure resin stone can do – and include results from my 5 µm sharpening stone here:
SEM micrographs of a M398 edge finished with Dr. Marvs Scientific sharpening stone, 5 µm. Instrument: Zeiss GeminiSEM 560.
It has a slightly cleaner, less wavy apex line. The surface in the SEM is comparable to the matrix stones. Optically, it’s much more homogeneous:
Optical micrograph of the M398 blade finished with Dr. Marv’s 5 µm stone. Instrument: Marvscope
In the 3D height map, we can identify fewer scratches and an overall smoother surface:
The surface roughness parameters are lower:
| Sa | 0.006707 | µm |
| Sq | 0.008277 | µm |
| Ssk | 0.2270 | – |
| Sku | 2.808 | – |
ISO 25178 surface roughness parameters. S-Filter: 2.5 µm (gaussian), L Filter: 0.25 mm (gaussian). No F operation besides LSQ leveling.
So, with a cheeky smile from my side: if you are looking to upgrade your Diamond Matrix 4000, I wouldn’t. It’s a fantastic stone. The new DiamondMax doesn’t differ significantly but in price. My 5 µm stone gives a cleaner result, but instead of buying my stone, I have a different suggestion for you:
Spend that “upgrade money” on a nice dinner with a person who is important to you. It is better invested.
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