This line has a measured particle size, rare for a natural stone. The microns come from published measurement rather than a grit-number estimate, so they are drawn at full strength, not faded.
Each grade below shows its size in microns, its spread (the published distribution, where a maker or standard gives one; most makers publish none), and where that size lands on the reference scale, a shared yardstick for reading grit numbers across makers.
Particle sizemicrons, log
10050302010
coarserfiner
inkmeasured or standardmaker figureour estimate
shapeknown distributionpoint, spread unknown
40 gritStandard-defined
428 µm
Conforms to ANSI B74.18 / CAMI; micron is the standard d50.