The Mohs scale ranks minerals from 1 (talc, the softest) to 10 (diamond, the hardest) by their resistance to scratching. This reference gives the defining mineral at each of the ten levels, the approximate absolute (Vickers) hardness, and everyday objects you can use to test an unknown sample in the field.
How it works
The Mohs scale is ordinal: a higher-numbered mineral scratches any lower-numbered one, but the steps are not equal in absolute terms. The rule for comparing two materials is:
if A_mohs > B_mohs → A scratches B
if A_mohs == B_mohs → they may just barely scratch each other
if A_mohs < B_mohs → B scratches AThe approximate Vickers column shows how non-linear the scale is — the gap from corundum (9) to diamond (10) dwarfs every other step.
The full ten levels
| Mohs | Reference mineral | Notes |
|---|---|---|
| 1 | Talc | Can be scratched by a fingernail; used in cosmetics |
| 2 | Gypsum | Barely scratched by a fingernail; includes selenite |
| 3 | Calcite | Scratched by a copper coin; found in marble and limestone |
| 4 | Fluorite | Easy to scratch with a steel knife |
| 5 | Apatite | Just scratched by a knife; apatite forms tooth enamel |
| 6 | Orthoclase feldspar | Scratches glass; common in granite |
| 7 | Quartz | Scratches steel; the threshold for “hard” gemstones |
| 8 | Topaz | Much harder than quartz by absolute measure |
| 9 | Corundum | Ruby and sapphire; extremely hard, used in abrasives |
| 10 | Diamond | The hardest natural material; far harder than 9 |
The jump from 9 to 10 is enormous in absolute terms. Diamond’s Vickers hardness is roughly 1,500 times greater than talc’s, but on the ordinal Mohs scale it is simply one step above corundum.
Field test objects
You can bracket an unknown mineral’s hardness with common items:
- Fingernail — about 2.5
- Copper coin — about 3.5
- Steel knife / nail — about 5.5
- Glass plate — about 5.5 to 6
- Steel file — about 6.5
How to test: Drag the test object firmly across a fresh surface of the unknown mineral. If a true scratch remains (not powder that rubs off, which can be the softer object abrading), the test object is harder. Work up from soft: if your fingernail scratches it, it is below 2.5; if a copper coin scratches it but a fingernail does not, it is between 2.5 and 3.5, and so on.
Why Mohs matters in practice
Jewellery durability. Hardness determines how a stone holds up to everyday wear. Anything below 7 (quartz) will be scratched by household dust, which contains quartz particles. Emerald is 7.5-8, aquamarine and topaz around 8, ruby and sapphire at 9. Softer stones like opal (5.5-6.5) and turquoise (5-6) need protective settings and gentler care.
Geology and mineral identification. In the field, a hardness test quickly narrows what a mineral could be. Combined with cleavage, luster, and color, hardness eliminates most candidates without lab equipment.
Engineering materials. Mohs values appear in abrasive and tooling selection. Cutting tools must be harder than the material they cut — a steel drill (roughly 6-7 Mohs) works for wood and soft rock but needs carbide or diamond coatings for harder materials.