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Mineralogy and cutting speed of a La Veinette natural combo


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Hello all -

Curiosity got the better of me and I had a look at the mineralogy of my La Veinette combo. I was unable to measure the entire bulk rock, so instead, I raised a slurry of comparable thickness (milkiness) from both the coticule and the BBW side of the hone. The slurry was air-dried and then measured using x-ray diffraction (XRD).


Mineralogy of the slurry raised from the BBW and coticule is ~similar, with the following exceptions:

1) the BBW has a higher content of (microcrystalline) quartz, mica, hematite, and chlorite, and
2) the coticule is dominated by garnet > mica > quartz with no hematite, and only minor chlorite.

The garnet content of the coticule slurry is considerably higher than that of the BBW. Note that this is for the slurry and not the bulk rock. Still, the lack of much garnet in the BBW slurry was surprising because BBW rocks are reported to have up to ~25% garnet vs. up to ~40% for the coticule (see, e.g.,
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). The low garnet concentration in the BBW slurry suggests that it may not be the actual bulk garnet content of the hone that is important, but rather the rate at which it releases garnets to slurry. At least in the case of my combo, the coticule releases its garnets much more readily than does the BBW. The data shown here is also consistent with my own observation that it is more difficult to raise a slurry on the BBW than on the coticule – or at least it takes longer to develop a slurry of equal concentration (milkiness). This effect is probably due to the (cementing) nature of the minerals that comprise the rock matrix (higher quartz, mica, and chlorite content, and the presence of hematite in the BBW). The trace Mn-oxide phases are of little consequence here.

In any event, I show here the raw data (as x-ray diffractograms) for your own interpretation. The scan for the BBW slurry is blue (top scan) and the red scan is the coticule (note that the scans were offset along the y-axis for clarity). The positions of ‘pure phase’ quartz and spessartine peaks (pink and blue, respectively) are shown as vertical lines on the bottom of the figure. Note that even pure phases have many peaks, and that peaks sometimes overlap among multiple phases. Only spessartine and quartz are shown here so that important relationships are not obscured by other phases.


I have labeled selected peaks for garnet (G), mica (M), chlorite (Ch), quartz (Q), and hematite (H). I think you can see the higher relative quartz and mica contents in the BBW, and the near absense of chlorite and hematite in the coticule, for example. You can also see the difference in garnet content between materials, but please note that most peaks (for all phases) are attributable to more than one mineral. For example, the garnet (G) peaks at ~48.2° and ~°59.5 in the coticule are due only to garnet. These same locations in the BBW scan have only a very small peak there (only minor garnet). On the other hand, the ‘garnet (G)’ peak on the coti scan at ~42.5° is caused by both garnet and quartz, so the BBW peak at this position is due to quartz alone and not garnet.

Anyway….this is just FYI….


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Could it be that in your BBW (and maybe in general), the primary abrasive is quartz and not garnet? Consider as well that, when used without slurry, BBW's are consider very slow.

Thank you for the analysis. It was a very interesting read!


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I suppose it's possible, but I think it's not very important in the long run (?). True, the quartz has slightly less hardness than the garnet, but it's micro- (or crypto-) crystalline (<<0.1u) in these materials (recrystallized as fine-grained, poorly-ordered material after the original sediments were deposited). As such, I would guess that it is probably far less abrasive than the garnets which are on the order of ~15-25u in size if I recall Bart's post on this.


Well-Known Member

this is great research. I've been on the lookout for your report ever since you told me about your intentions to run an X-ray diffraction on your combo stone. For your convenience, I will post mineralogical data from the book recently published by the Geological Department in Belgian National Science institute. Give me some time to figure out which data corresponds to yours and to type it out.

Concerning the low garnet content of your BBW, we must be careful not to jump conclusions. In the old days, only the BBW of very specific BBW-layers from the Ol'Preu deposit were sold for honing puproses, under the name "Pierre Lorraine". The Blue stone from other deposits near the Salmchateu, Ottre and Hébronval mining locations were considered "sterile" (meaning without much use).
One of the reasons why Ardennes doesn't charge extra for a Coticule that's still bonded to it's blue counterpart, is that there's no guarantee about the abrasive properties of that Blue. Since they all come from Ol'Preu, a deposit that was known for "good" BBW layers, we notice in practice that it are mostly good hones, albeit quite slow.

Your hypothesis that BBW relies on more than just its garnets for its abrasive capabilities is very interesting, and I think you might be on to something here. There sure are indications that weird things are going on with the BBWs we're testing in the "Operation True Blue" experiment (run by the Research Team of We've not yet formulated conclusions, but I can reveal already that the BBW can be used as a finishing hone: it both can reach the necessary keenness without any help of other stones and it can leave an edge smooth enough to rival with many reputed finishing hones. How it achieves that with its larger garnets has been a mystery to me, although I have formulated a plausible hypothesis, by assuming that the larger garnets have more facets than the smaller ones and are therefor "smoother" cutters. But that does not explain why we don't see "slurry-dulling" on a BBW as we see it on Coticules.

If I'm informed correctly, hematite is what gives the BBW its color, so I'm not surprised that you found it not in the Coticule slurry.

If you would like to run more tests, I can provide you with samples that may be destroyed.

Thanks again,


Well-Known Member
Hi Bart –

Thanks for the note. I look forward to seeing the data from the Belgian Nat’l Sci. Inst. Should be very interesting.

I fully realize that the weak link in the chain here is the absence of any data for the bulk rock(s). More importantly, I certainly don’t mean to imply in any way that the (my) BBW is ‘sterile’. In fact, I personally do not believe that to be the case. Whether the lower garnet content of my BBW slurry is the result of low inherent garnet content in the bulk rock, or to slow release rates from the rock is unknown. The former seems unlikely, and I also defer to your considerable experience in these matters.

But, yes – the BBW is slow, and whether that is due to garnet content, or to garnet release is unknown. It’s probably not an ‘either/or’ situation, but we can test that later.

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I can see where larger garnets=smaller surface area of abrasive (garnet) material and therefore less cutting. One other possibility is that the BBW slurry has ~15% higher phyllosilicate content than the coti, and of course, these are some of the principal components in many Jnats. Phyllosilicates in the slurry may well break down to finer particles sizes during honing just as they do in the Jnats.

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I am cautious about this one. None of the minerals identified really do it for me! Hematite (Fe[sup]3+[/sup][sub]2[/sub]O[sub]3[/sub]) is typically very highly oxidized and deep red. The deep blue color of the BBW has me puzzled.

In any event, I wouldn't be opposed to having another go at this. I used to have daily access to the equipment I need, but nowadays, it's much more sporadic. The instrument that I now have access to cannot be configured for larger (irregularly-shaped) samples (hones), so to do the work I need powders or thinner rock samples, say ~2cm x 2cm x 0.3cm (thick). What might work would be for you to send me 4 samples:

1) Ground BBW (powder)
2) Ground Coti (powder) - both of these preferrably from a natural combo?
3) Dried Slurry from (1)
4) Dried Slurry from (2) - both slurries of ~same consistency.

We can talk about sample prep if we move forward.

OK - that's about. Let me know what you think.
I appreciate your thoughts.



Well-Known Member
Hi Steve,

Thanks for the very interesting post. You have all the necessary expertise and access to scientific instruments. Perhaps it is possible to solve one more mysterious question about coticule with your help?
If we compare the slow and fast coticule. Then there is a very interesting question - whether coticule speed depends directly on the garnet content or there is other reason that makes coticule fast?

Best regards,


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Thanks, Urmas. As I said, my access to the equipment is somewhat more limited these days, but I'm happy to see what can be done.



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Woodash said:
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Hello Steve,

This is truly a unique opportunity to learn the inner difference between slow and fast coticule.
I have a slow coticule from La Verte layer, which is likely much slower than your La Veinette - this will be an interesting comparison.
I can create the slurry and dry it and send it by letter to you to investigate it. How much of dried slurry would be needed and whether there are any additional conditions? I think that I will use a DMT diamond hone to rise a slurry, it probably led to increased nickel content in analysis result, but I think that is only possibility to avoid any influence on the results by slurry stone.

Thank you very much,