The Mohs Hardness Scale (With Examples)
The Mohs hardness scale is a simple way to measure how resistant a mineral is to being scratched. Devised in 1812 by the German mineralogist Friedrich Mohs, it ranks ten reference minerals from 1 (the softest, talc) to 10 (the hardest, diamond). The rule is straightforward: a harder mineral can scratch a softer one, but not the other way around. Because you only need the mineral itself and a few common objects, hardness is one of the most useful field tests for identifying an unknown rock, crystal, or gemstone.
One thing to keep in mind is that the Mohs scale is ordinal, not linear. The numbers tell you the order of hardness, not how much harder one mineral is than another. The jump from corundum (9) to diamond (10) is enormous — diamond is several times harder than corundum, far more than the single step between them suggests, while the gaps between the lower numbers are much smaller and more even. So use Mohs to rank and narrow down, not to calculate exact differences.
How the scale works
Mohs hardness measures scratch resistance — specifically, a mineral's resistance to having its surface permanently scratched by another material. It is a relative, comparative scale built around ten well-known reference minerals, each assigned a whole number from 1 to 10. To place an unknown specimen, you find which reference minerals (or everyday substitutes) it can and cannot scratch, and read its hardness from where it falls.
If two materials scratch each other only with difficulty, or not at all, they have roughly the same hardness. Hardness is not the same as toughness or durability: a diamond is extremely hard but can still shatter from a sharp blow, and a tough mineral like jade resists breaking yet is only moderately hard. Mohs hardness tells you about scratching, and nothing more.
The 10 Mohs reference minerals
| Hardness | Mineral | Notes |
|---|---|---|
| 1 | Talc | Softest mineral; greasy feel, easily scratched by a fingernail and crushed to powder (talcum). |
| 2 | Gypsum | Can just be scratched by a fingernail; the source of plaster and alabaster. |
| 3 | Calcite | Scratched by a copper coin; fizzes in dilute acid, a separate diagnostic test. |
| 4 | Fluorite | Scratched by a knife; shows perfect cleavage and often glows under UV light. |
| 5 | Apatite | The reference for 5; about the hardness of tooth enamel and a steel knife blade. |
| 6 | Orthoclase feldspar | Will scratch glass; a very common rock-forming feldspar. |
| 7 | Quartz | Scratches glass and steel easily; the benchmark for a 'hard' mineral and the main mineral in sand. |
| 8 | Topaz | A hard gemstone with perfect cleavage; scratches quartz. |
| 9 | Corundum | The mineral of ruby and sapphire; scratches topaz and is used as an abrasive. |
| 10 | Diamond | The hardest known natural mineral; scratches everything else and nothing natural scratches it. |
Everyday objects you can test with
You do not need a boxed set of reference minerals to estimate hardness. A handful of common objects covers most of the useful range, letting you bracket an unknown specimen between two known values.
| Object | Approx. hardness |
|---|---|
| Fingernail | 2.5 |
| Copper penny (pre-1982 US cent) | 3.5 |
| Iron nail / knife blade | 5 to 5.5 |
| Glass plate | 5.5 |
| Steel file | 6.5 |
| Streak plate (unglazed porcelain) | 7 |
How to do a scratch test
A scratch test is destructive — it leaves a permanent mark — so always test on an inconspicuous spot or a small broken edge, never on a polished face, a gem setting, or a display-quality crystal. Work over a soft surface so you do not drop the specimen, and wipe the area afterward to be sure you are looking at a real groove and not a smear.
- Find a fresh, flat, unweathered surface on the unknown specimen — a clean broken edge is ideal.
- Choose a tester of known hardness (a fingernail, coin, knife, glass, or a reference mineral) and press its point or edge firmly across that surface in a single short stroke.
- Wipe away any powder and look closely: a true scratch is an actual groove cut into the mineral, not a removable streak of material left on top.
- If the tester does not scratch the specimen, move up to a harder tester; if it does scratch it, move down to a softer one, until you bracket the specimen between something that scratches it and something that does not.
- Record the result as a range — for example, 'scratches glass (5.5) but is scratched by a steel file (6.5),' giving a hardness of about 6.
Why hardness matters for identification
Hardness is powerful because it cleanly separates look-alikes that color and shape alone cannot. Clear calcite and clear quartz can look almost identical, but calcite is a 3 (a knife scratches it easily) while quartz is a 7 (it scratches the knife instead) — one quick test settles it. Pyrite ('fool's gold,' about 6 to 6.5) is far harder than real gold (only 2.5 to 3), so a knife edge that bites into gold simply skates off pyrite.
The same logic flags fakes. A genuine gemstone such as sapphire (corundum, 9) or topaz (8) will scratch ordinary glass with ease, whereas a glass imitation (about 5.5) will not scratch a glass plate and is itself scratched by quartz and steel. Hardness rarely gives a final answer on its own, but combined with luster, streak, cleavage, and crystal form it quickly narrows a mystery rock down to a short, testable list.
Frequently asked questions
What is the hardest mineral on the Mohs scale?
Diamond, at 10, is the hardest mineral on the Mohs scale and the hardest known natural material. It can scratch every other mineral, and nothing found in nature can scratch it. Corundum (ruby and sapphire) is next at 9, but the gap between 9 and 10 is far larger than the single step implies — diamond is several times harder than corundum.
Is the Mohs scale linear?
No. The Mohs scale is ordinal, meaning it only ranks minerals from softest to hardest; it does not measure how much harder one is than another. The intervals are uneven — the step from 9 (corundum) to 10 (diamond) represents a much bigger jump in actual hardness than the step from, say, 2 to 3. Use it to rank and compare, not to calculate exact ratios.
How hard is a fingernail, a knife, and glass on the Mohs scale?
A fingernail is about 2.5, a steel knife blade or iron nail is roughly 5 to 5.5, and a glass plate is about 5.5. A copper penny sits near 3.5 and a steel file near 6.5. These everyday objects let you bracket an unknown specimen's hardness without owning a set of reference minerals.
How do I test a mineral's hardness at home?
Pick a fresh surface on the specimen and try to scratch it with objects of known hardness — a fingernail (2.5), copper penny (3.5), knife or nail (about 5.5), glass (5.5), and steel file (6.5). Wipe off any powder and confirm you see a real groove, not a removable smear. Work up or down in hardness until you find what does and does not scratch it. Because the test leaves a mark, use a hidden spot or a broken edge.
What is the difference between hardness and toughness?
Hardness is resistance to scratching, which is what the Mohs scale measures. Toughness is resistance to breaking, chipping, or shattering — a different property entirely. Diamond is the hardest mineral but is not especially tough and can fracture from a sharp blow, while jade is only moderately hard yet extremely tough. A mineral can be hard, tough, both, or neither.
Related guides
Last updated 2026-06-25. Identification guidance is educational — confirm important results with the tests described or a qualified expert.