Whetstone or Diamond? Which Knife Steels Need a Diamond Stone

01 · IT’S NOT HARDNESS (HRC) — IT’S THE CARBIDES

It feels obvious that a harder knife should be harder to sharpen. But HRC measures the whole blade’s resistance to denting — not how easily a stone removes metal. A high-carbon White steel at HRC 64 sharpens beautifully, while some softer “super steels” at HRC 60 barely budge on the same stone.

What actually resists your stone are carbides: tiny, very hard particles scattered through the steel like raisins in dough. Two things about them decide your experience. ① How much carbide there is — more carbide means more passes to grind the same edge. ② How hard the carbide is — if it is harder than your stone’s abrasive, the stone can’t cut it cleanly and you need diamond.

02 · TWO DIFFERENT KINDS OF “HARD TO SHARPEN”

Before the map, it helps to split the complaint in two, because they have opposite causes. ① “It won’t grind down.” The metal removes slowly — that’s wear resistance, i.e. carbide amount × hardness, and it’s what the map below is about. ② “It won’t take a clean edge / it stays gummy.” A soft, ductile steel folds into a stubborn burr instead of crisping up — that’s ductility, not carbide, and it’s a technique fix, not a stone fix. If that’s your problem, see how to remove the burr and why a knife won’t get sharp.

03 · THE MAP: CARBIDE AMOUNT × HARDNESS

Here is every common knife steel on those two axes. Left–right is how much carbide there is (more passes); up–down is how hard the carbide is (whether a normal stone can cut it). The dashed red line is the threshold: carbides above it are harder than common abrasives, so the finish needs diamond. Everything below it sharpens on a normal whetstone.

Three things jump out. ① HRC barely predicts it — three steels here are HRC 65 (Aogami Super, ZDP-189, HAP40) yet land in completely different zones, and the single hardest to sharpen, S90V, is “only” HRC 60. ② “Hard to sharpen” is not “needs diamond” — ZDP-189 takes many passes but its carbide is chromium-type, softer than your stone, so a whetstone still works. ③ The diamond group is small — mostly high-end folder steels (S30V, M390, S90V) and a few premium Japanese steels (SG2/R2, HAP40), and only for the fine finishing step.

04 · HOW A WHETSTONE CUTS A CARBIDE HARDER THAN ITSELF

Here is the part that confuses people. If vanadium carbide is harder than your alumina stone, how does any stone touch a steel that contains it? The stone doesn’t have to cut the carbide — it undermines the softer steel around it and pops the whole grain out, like digging out a raisin by removing the dough around it.

That is why coarse and medium stones handle almost anything: the grit is bigger than the carbides, so it tears out metal and carbide together. The trouble only starts at fine grits, where the abrasive becomes smaller than the carbides and has to cut through them. That is the exact step where high-vanadium steels stall and a diamond finish earns its keep.

05 · THE VANADIUM TRAP — SPEC-SHEET V ISN’T VANADIUM CARBIDE

This is where most buying advice goes wrong. A spec sheet might list 2% vanadium and people assume “needs diamond.” But vanadium only becomes hard vanadium carbide (VC) if there is spare carbon to form it. In high-chromium steels the chromium grabs most of the carbon first — so the actual VC is tiny. That is why VG10, SG2/R2 and ZDP-189 all sharpen on a normal whetstone despite their reputation: they are chromium-carbide steels. True diamond territory starts only when there is a lot of real VC — the high-vanadium tool steels.

06 · “SLIPPERY” ISN’T “THE STONE CAN’T CUT IT”

People call VG10 and powder steels “hard to sharpen” because they feel slippery on the stone. That is usually bite, not abrasion: a fine, dense steel raises little slurry, so the edge skates instead of grabbing. The fix isn’t a harder stone — it’s a bit of slurry. A light pass with a nagura, or a slightly softer stone that releases grit, restores the grab and the steel cuts normally. So keep two questions separate: can my stone abrade this steel? (almost always yes) and does the edge bite the stone? (a slurry and feel issue). A plain stainless knife, for instance, sharpens fine on one #1000 stone.

07 · STEEL DICTIONARY — FIND YOUR KNIFE

Grouped the same way as the map, by the hardest carbide each steel contains. HRC is a typical range and shifts with heat treatment.

A. Carbon steels (iron carbide) — easiest, finest edge.
Shirogami / White (#1, #2) — HRC 60–64. No alloy carbide; the keenest edge of all. Rusts easily.
Aogami / Blue (#2) — HRC 61–64. White plus a little Cr and W; better holding, still very easy.
Aogami Super (Blue Super) — HRC 64–66. High carbon with W/V; harder and more passes, but mostly iron carbide, so stones cut it. Top-tier edge and retention.
Yellow / SK (SK4, SK5) — HRC 58–62. Inexpensive carbon steel; grinds easily. Rusts.

B. Stainless — chromium carbide (whetstone is fine).
Molybdenum stainless / X50CrMoV15 (4116) — HRC 55–58. The everyday mass-market kitchen stainless; soft and forgiving.
8Cr13MoV / 9Cr18MoV — HRC 57–60. Budget stainless; sharpens easily.
AUS-8 / AUS-10 — HRC 57–60. Workhorse stainless; straightforward.
14C28N (Sandvik / AEB-L family) — HRC 58–62. Fine grain, very clean edge; easy and rust-resistant.
Ginsan (Silver-3) — HRC 58–61. Sharpens almost like carbon steel; a great Japanese stainless for beginners.
VG10 / VG1 — HRC 60–61. The classic “first wall”: fine edge, good retention, can feel slippery — but a whetstone handles it.
ATS-34 / 154CM / CPM-154 — HRC 59–61. 14Cr-4Mo, no vanadium; a Western-knife staple. Stones fine.
440A / 440C — HRC 56–60. 440C has larger chromium carbides: more retention, slightly coarser edge.
D2 / SKD11 — HRC 60–62. Big chromium carbides, high wear resistance; more passes but stones work. Semi-stainless.
ZDP-189 — HRC 64–67. A huge volume of chromium carbide; extreme edge retention. Hard work, but no diamond needed.

C. Vanadium steels — diamond helps or is required.
SG2 / R2 (SGPS) — HRC 62–64. Premium PM stainless; ~2% V but low actual VC, so stones work — diamond just refines faster. Common on high-end Japanese knives.
S30V / S35VN — HRC 58–61. ~4% V with real VC; a diamond finish is noticeably easier. Mostly premium folders.
M390 / 20CV / 204P — HRC 60–62. High chromium plus ~4% V; premium folder steel; diamond to finish.
HAP40 (PM high-speed steel) — HRC 64–67. Low chromium, so real VC forms; superb retention on premium Japanese knives; diamond to finish.
MagnaCut — HRC 60–64. Fine V/Nb carbides — easier to sharpen than its wear resistance suggests.
S90V / S110V / CPM-10V / K390 — HRC 58–62. Lots of large vanadium carbide; diamond or CBN is effectively required.

Not a steel: ceramic blades (zirconia) — diamond only, and lightly: ceramic is brittle and chips, so it is as much about avoiding damage as removing material.

08 · AT A GLANCE

09 · SO WHAT SHOULD YOU ACTUALLY BUY?

If you own normal kitchen knives — Japanese or Western, stainless or carbon — a single medium #1000 whetstone does almost everything, including the steels people think need diamond (VG10, SG2). New to stones? See what grit you actually need, how to sharpen on a whetstone, and how to hold a steady angle. (And if you’re tempted by a gadget, here’s whetstone vs pull-through.)

Consider diamond only if you own and want to mirror-finish a high-vanadium steel (S30V, M390, S90V, HAP40). Even then you shape and set the edge on normal stones and switch to diamond only for the last refining step — and because electroplated diamond plates run coarse-to-medium, a true fine finish means a resin-bond diamond stone or a diamond/CBN strop.

FAQ

Q. Is a harder knife always harder to sharpen?

No. HRC and sharpening difficulty correlate weakly. The carbides (amount × hardness) decide it. An HRC 65 steel can sharpen on stones, while a mid-hardness one (S90V) can need diamond.

Q. Do I need a diamond stone for VG10, SG2 or ZDP-189?

No. All three are chromium-carbide steels, so a normal whetstone cuts them. They can feel slippery; that’s about bite, not your stone being too soft.

Q. Why does my premium knife feel slippery on the stone?

Dense, fine steels raise little slurry, so the edge skates. Raise some slurry with a nagura or use a stone that releases grit and the bite comes back.

Q. Which knives genuinely need diamond?

High-vanadium “super steels” (S30V, M390, S90V, HAP40 and similar) for a fine finish — plus ceramic blades. Everyday kitchen stainless and carbon steels do not.

Sources & method: carbide types, hardness (Knoop) and volume estimates are based on the research of metallurgist Larrin Thomas at Knife Steel Nerds (see also VG10 & SG2). Abrasive hardness (Knoop): vanadium carbide ≈2800, silicon carbide ≈2600, alumina ≈2100. Map positions are relative estimates from steel composition, not to scale; HRC and carbide volume vary with heat treatment.