How to Tell the Grit of a Sharpening Stone

You can place an unmarked stone in a grit band in about five minutes. You cannot establish its number, and nobody can do that at home. The band is what actually matters: coarse (under roughly 400) for repair, medium (800–1200) for real sharpening, finishing (3000–6000) for refining, and polishing (8000 and up). Knowing which of those four you are holding is enough to use the stone safely.

First, what the numbers mean at all. A grit designation describes the abrasive particle size — higher number, finer particles, finer scratch pattern. Japanese water stones run from a few hundred up past 8000. The numbering is not standardized across countries or abrasive families, which is why a 1000-grit Japanese water stone and a 1000-mesh diamond plate do not leave the same finish. For a reference point that is published, DMT gives its own grades as Coarse at 325 mesh / 45 micron, Fine at 600 / 25 micron and Extra-Fine at 1200 / 9 micron.

Test 1: the marker test (the most informative one)

Color the bevel of a knife you do not love with a permanent marker, wet the stone, and take three light passes.

• Ink gone in one or two passes, with visible metal swarf— coarse. Under about 400.
• Ink gone in three to six passes, grey slurry forming— medium. Somewhere in the 800–1200 band, which is the workhorse range.
• Ink fading slowly over a dozen passes, almost no swarf— finishing, 3000–6000.
• Ink barely marked after twenty passes, surface feels slick — polishing, 8000 and up.

This works because cutting rate is the practical definition of grit. It is the same marker trick used to check sharpening angle, described in our troubleshooting guide, applied to a different question.

Test 2: feel the surface, dry

Dry, with a clean fingertip, drag across the face rather than along it. A coarse stone feels like a sidewalk — you can feel individual particles catch. A medium stone feels like fine sandpaper: uniform tooth, no individual grains. A finishing stone feels smooth with a faint drag, like unglazed porcelain. A polishing stone feels like glass with a slight grip. This is crude and surprisingly reliable at separating the four bands.

Test 3: what it does with water

This identifies the family, which correlates loosely with grit. There are two kinds of water stone: soaking stones that must be submerged until saturated, and splash-and-go stones that need only surface water. One maker’s own range splits exactly that way — its Traditional, Aotoishi, Lobster and Hibiki lines are “soaked for several minutes” while its Chocera Pro, S1/S2 Advance and diamond stones are splash-and-go.

• Drinks water audibly, bubbles, darkens— a porous traditional soaker. These cluster coarse-to-medium, and the classic learner’s stones are in this group.
• Water beads or sits on the surface— a dense, hard formulation, typically a harder modern synthetic, and more often fine than coarse.
• Water will not wet it at all and it smells faintly of oil— it is an oil stone, not a water stone. Do not soak it, and do not expect water stone behaviour; it wants honing oil.

Our soaking guide covers why soaking the wrong stone is a genuine way to crack it.

Test 4: read the scratch pattern on the bevel

After the marker test, tilt the bevel to a light. Coarse stones leave scratches you can see with the naked eye and a bevel that looks matte grey. Medium stones leave a uniform satin finish. Finishing stones leave a bevel in which you can start to see a reflection. A polishing stone leaves something close to a mirror. Compare two unknown stones against each other this way and you can rank them confidently even when you cannot number either.

Test 5: slurry color and amount

Work the stone wet for thirty seconds. A coarse stone throws a thick dark slurry quickly — that is steel plus released abrasive. A medium stone builds a thinner grey slurry. Fine stones produce very little, and what there is looks pale. A stone that produces no slurry and no feedback at all is either very fine or glazed over, which brings us to the last check.

Test 6: is it flat, and is it glazed?

Before trusting any of the above, establish that the stone is usable. Stones dish in the middle with use, and a hollow stone grinds a convex bevel whatever its grit. Draw a pencil grid across the face and take a few passes on a flattening plate: if the grid survives in the middle, the stone is dished. Our flattening walkthrough is the fix, and it also removes glazing — a polished, metal-loaded surface that makes a medium stone behave like a fine one and will throw every test above.

What none of this will tell you

A number. Treat any specific figure you arrive at as a guess. Band yes; number no.

Whether it is natural or synthetic, in many cases — and with natural stones the grit varies within one stone.

Whether it is safe for your best knife. This is the part worth being conservative about. If a mystery stone turns out to be coarser than you assumed, you will take significantly more steel off a thin 60 HRC Japanese edge than you meant to. Test on a cheap knife, and if the stone is dished, cracked or oil-soaked, the honest answer is that a new marked stone costs less than the knife you are risking. The three below all print their grit on the stone, which is the whole point.