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:
Part 1 is about the Fällkniven DC3 ,
Part 2 is about the DMT mini W7C,
Part 3 is about the TSPROF Blitz F1000.
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.
Today’s sharpening stone is a natural jade stone. Jade is a natural occurring stone, that consists out of complex silicates. I believe that typically the one used for sharpening is from the pyroxene group. There, the minerals consist out of a certain formula: (XY(Si,Al)2O6) where X typically is a light metal such as Calcium, Sodium and Y typically is a heavier metal such as aluminium, chromium. I mention this, because jadeite typically is not pure NaAlSi2O6, but more likely a wild and varying mix of several elements, some maybe only in traces. Nevertheless, silicates are hard, with jadeite typically reaching around 1000 HV. This is harder than a cheap knife, but actually softer than some high carbide steels at their maximum achievable hardness. The other mineral that is commonly called jade is nephrite, which is a really complex mix of Ca2(Mg,Fe)5Si8O22(OH)2. This is typically a bit softer, ranging from 700 to 1000 HV (hardness vickers). We’ll see whether we can identify what mineral our natural jade stone is made of later!
Taking a look under the optical microscope, it is clearly identifiable as jade by it’s distinct, swirly white and green colour:
Optical micrographs of a natural jade stone. The scale bar is visible in the lower right corner. Instrument: Leica Emspira.
One can make out some distinct silicate grains, with a more whitish colour, and also some darker debris, likely swarf stuck to the surface. Jadeite is non-conducting, so we once again get to enjoy absurdly detailed pictures from the fantastic Zeiss GeminiSEM560. Because of it’s design, it excels at low voltage imaging, where one has lower charging effects.
SEM Micrographs of the surface morphology of the natural jade stone. Instrument: Zeiss GeminiSEM560.
The SEM pictures reveal a ragged topography, consisting out of a mix of ultra fine, debris like grains, but also larger, well formed grains. I struggle to give this stone a classification in terms of grit – the grains are sometimes in the nanometer range (compare the 2KX magnification picture), but some are also several micrometre large, up to maybe the low double digits.
A nice question here is: what type of jade is this actually made out of? For this, we employ the EDS module of the Zeiss GeminiSEM560. With energy dispersive x-ray spectroscopy (typically abreviated EDS or EDX), one can identify the elemental composition of a SEM sample. This doesn’t mean you press a button and crime show like you get a beep and it identifies the material. It means that after a couple minutes, with the uncertainty of a couple percent, you can state “I think it contains iron. maybe.”. Welcome to science!
EDS analysis of the natural jade stone. Instrument: Oxford Ultim Max ∞ 40mm2 EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.
It’s generally a good idea with EDS analysis to compare the composition in percent (visible in the second picture above) with the colour map of where these elements appear. As the “full colour” summary image is quite hard to differentiate, I typically use that one to pick out the area or grain of interest, and then peek to the smaller, individual colour slices. EDS analysis nicely identifies that this jade stone consists out of Ca, Mg, Si, O. These are the elements found in neprhite (Ca2(Mg,Fe)5Si8O22(OH)2), so it’s pretty safe to say this is what the stone is made out of!
One really cool thing our Zeiss is equipped with is a sensor called “VPSE”. This stands for variable-pressure, secondary electron detector. It doesn’t detect electrons directly, like a Everhart-Thornley would. If your sample is in “low vacuum” conditions, that ET-SE detector would short circuit out because the air is conducting. Nevertheless, secondary electrons are created by the electron beam-matter interaction, and those create little light flashes when they hit molecules in the low atmosphere at VP. Now, the VPSE detector has a very sensitive scintillator that detects these flashes. I’m telling you this, because if you aren’t in low vacuum, and you have a mineral sample, you can use the VPSE as a cathodoluminescence detector. Here, the electron beam sometimes creates light in the interaction with certain minerals. Sadly, with “abusing” the sensor in this way, it’s still only a black and white picture. But if your sample glows, you can make that visible.
“abused” VPSE sensor micrograph to highlight cathodoluminescence of the jade stone. Image FOV is identical to the SE2 image earlier in this post. Instrument: Zeiss GeminiSEM560.
Does it have any relevancy to this post? No. Is it super awesome and cool? Yes!
In order to look at the surface of the stone, the awesome Zygo Nexview NX2 white light interferometer comes to use again:
The jade stone while being measured on the Zygo Nexview NX2 interferometer.
The stone shows a relatively smooth surface. The uppermost surface is actually pretty flat, with a large material ratio (bearing surface). Some deep voids are sprinkled randomly over the surface.
White light interferometry height map of the jade stone. Instrument: Zygo Nexview NX2, Objective Lens: 10X. Stitched overview of 5×5 images.
The high material ratio with a smooth surface is one reason why this stone feels nearly like glass – very little feedback, as the contact surface area is large, and the knife slides along it without really grabbing onto the grains. This is a stone for very low removal and mostly I would guess it burnishes a knife edge.
ISO 25178 parameters of the natural jade stone.
Roughness wise, this is a pretty coarse stone (Sq > 7 µm), with a sharp profile (Sku, kurtosis >> 3). Nevertheless, the topmost surface is ground pretty flat.
Leave a Reply