This is a chart I reference frequently and is probably one of the best Rosetta stones available for deciphering the tower of Babel of grit rating measurements.
I tend to use microns as the most widely consistent means of translating all of this into a common methodology across the rating systems. Because of my bias I also think in terms of Japanese rating scales, which I believe is pretty consistent across most Japanese waterstones. This includes Shapton, Naniwa stones (all series), Nubatama, etc.
My compounds are simply stated in terms of microns, as well as my diamond films, secondarily translated into grits.
It is worth noting but then ignored that Shapton GlassStones are rated not in GRIT ratings but rather MESH ratings. There is a slight difference between grit and mesh rating systems, but it will just make this discussion even more convoluted, so let it RIP. Close enough but worth noting in your travels exploring 'grits'.
As BDL notes there is more to it than just the grit value assigned to a product, noting some of the variables involved in stone selection - "Type of binding substrate, hardness, type of abrasive".
Of course there are things other than stones to consider in this discussion too - belts, compounds, sprays, films, pastes, etc. Here too other factors become relevant - belt stiffness, makeup of suspension slurry and paste components and the type of particle and particle shapes and particle friability to name but a few more.
Back to just the grit numbers. What does this number mean? Well in the best of cases, it accurately reflects the AVERAGE particle size of the stone, slurry etc. The accuracy of this value DOES vary from manufacturer to manufacturer. Furthermore, it says nothing of the variance in particle size. For synthetic products, most particles should be all about the same size with a narrow variance. NO product has all particles exactly the same size. There is always some variance. Good products have narrow variances and certainly good products all vary about one particle size. So if a product has two or more particle sizes in it, this is indeed a poorly specified product. You also want your product to be a pure product - not containing unspecified stray ingredients not put there on purpose in unknown amounts. Sometimes a product for example some of the Nubatama stones will purposely add a mystery or secret ingredient to produce some of those intangible effects on purpose. Some stones - like the Han-Han are purposeful blends of natural and synthetic stone particles for instance.
Particle shape is also relevant. So if a particle is a perfect sphere, diameter is a straightforward measurement. Sometimes it is more football shaped. Do you measure the long axis short axis, or average length ignoring this variance? And so on with more and more complex shapes. Particles can be more spherical (CRO), more spherelike - dodecahedral shapes with 12 facets like coticules, etc. They can be simple shapes like a monocrystaline diamond or complex composite shapes like polycrystalline diamond with many facets providing significantly more cutting action for the same size particle. The points on a particle and their size also influence how deeply they cut, with smooth spheres cutting shallower grooves ... and so on.
Particle agglomeration is also quite important. If the particles stay separated from each other, then the particle size of each individual particle accurately reflects the effective grit of the product. But if they lump and stick together like bad gravy, these lumps act like larger particles and the result is a very inconsistent product. This lumping is called agglomeration and is to be avoided. This is relevant in slurries and I take great pains to avoid agglomeration in my CBN and diamond products. Some other manufacturers do not, either avoiding the issue or just being unaware of the issue. For this reason, home brews who just mix up some chromium oxide powder get massive agglomeration for instance, so buying powders and mixing them up yourself is going to give very poor results. One of the main causes of particle agglomeration is electrostatic forces. Think of the static cling when your socks stick to other clothes in your clothes dryer. One cause of this static buildup is the ions in your water. These charged particles increase the likelihood of agglomeration. The smaller the particle the greater the tendency to agglomerate. So if you are dealing with a 5 micron particle this tendency is low. At a quarter micron it is high. So with deionized water particle agglomeration is minimized. Particle density also increases the tendency to agglomerate. My compounds are high density preparations, but each product is analyzed for any tendency to agglomerate and carat concentration is strictly controlled.
Particle agglomeration in stones is also of concern. The development of stones like the 15k Nubatama needed to take this into account. Given it's rather unique formulation using a level of art far beyond my pay grade, I can only be amazed at the result, both comparing it to natural and synthetic stones. Stones in the 15k and above range require that particular attention be paid to this variable in their formulation are rare, for example the Shapton 15k and 30k stones. How agglomeration is controlled in synthetic stone formation is also a bit beyond me. You can also sometimes see agglomeration in stone slurries, but this is yet another topic.
Now with natural stones, things get even more complex if the particles are thin sheets or flakes like a corn flake. Do we measure length width or even thickness? And if the relevant criteria is the surface texture of the flake, now we are even in more trouble getting a meaningful measure. Now what if these flakes start breaking apart and changing as we use the product? Now we are in a near hopeless mess in terms of getting a meaningful measure. Further, the 'particles' aren't all of the same type or hardness, break down at differing rates and so on, further compounding accurate measures into something nearly useless. Please keep this in mind when asking what grit a particular natural stone is. Now if different particles cut differently the scratch pattern on softer steel will be different than on harder steel so the effective 'grit' becomes different depending on the steel that is being abraded. It is an estimate at best.
I go into more depth on the topic in a series of articles I wrote a few years ago comparing various preparations of diamond particles, showing both electron micrographs of the particles and particle size distribution data (PSD) for various products. Here's a link to this series of articles to get a more detailed understanding of what is behind just giving a number like 1/4 micron (64k grit) to a product.http://www.precisesharpening.blogspot.c ... arter.htmlhttp://www.precisesharpening.blogspot.c ... arter.htmlhttp://www.precisesharpening.blogspot.c ... nning.html
Feel free to ask questions about these here.