What the Mohs scale measures
Friedrich Mohs, a German mineralogist, introduced the scale in 1812. It is a scratch test: a mineral is harder than everything it can scratch and softer than everything that scratches it. If a specimen is scratched by apatite but not by fluorite, its hardness lies between 4 and 5.
The scale is ordinal, not proportional. The numbers give an order, not a measure. Measured with a sclerometer, corundum (9) is twice as hard as topaz (8), but diamond (10) is about four times as hard as corundum. The jump from 9 to 10 is larger than the whole distance from 1 to 9.
The ten reference minerals
Each step is defined by a common mineral. The last column shows the absolute hardness measured by sclerometer, which makes the uneven steps visible.
| Mohs | Reference mineral | Formula | Absolute hardness |
|---|---|---|---|
| 1 | Talc | Mg₃Si₄O₁₀(OH)₂ | 1 |
| 2 | Gypsum | CaSO₄·2H₂O | 2 |
| 3 | Calcite | CaCO₃ | 14 |
| 4 | Fluorite | CaF₂ | 21 |
| 5 | Apatite | Ca₅(PO₄)₃(OH,Cl,F) | 48 |
| 6 | Orthoclase feldspar | KAlSi₃O₈ | 72 |
| 7 | Quartz | SiO₂ | 100 |
| 8 | Topaz | Al₂SiO₄(OH,F)₂ | 200 |
| 9 | Corundum | Al₂O₃ | 400 |
| 10 | Diamond | C | 1500 |
Mohs hardness of popular gemstones
Gemstones are judged on the same scale. The harder the stone, the better it resists everyday wear in a ring; softer stones such as opal, turquoise and pearl scratch easily and are better set in pendants or earrings.
| Gemstone | Mohs hardness |
|---|---|
| Diamond | 10 |
| Moissanite | 9.5 |
| Ruby, sapphire (corundum) | 9 |
| Topaz | 8 |
| Emerald, aquamarine (beryl) | 7.5 to 8 |
| Garnet | 6.5 to 7.5 |
| Amethyst, citrine (quartz) | 7 |
| Jadeite jade | 6.5 to 7 |
| Opal | 5.5 to 6 |
| Turquoise | Just under 6 |
| Pearl | 2.5 to 4.5 |
A scratch test with things you already own
You do not need a hardness kit to place a mineral roughly. Everyday materials have known hardnesses:
- Fingernail: about 2.2 to 2.5. It scratches gypsum but not calcite.
- Copper: about 3.
- Window glass and ordinary steel, such as a knife blade: about 5.5.
- Hardened steel, such as a file: about 6.4.
- Quartz scratches glass; anything that scratches quartz is harder than 7.
How to do it without ruining a specimen
A scratch test damages what it touches, so never do it on the face of a specimen you care about.
- Test on the back, on a broken surface, or on a loose fragment.
- Press firmly and drag a few millimetres; do not hammer.
- Wipe the mark with a finger. A powder line that rubs off is the softer material leaving a trail on the harder one, not a scratch.
- Test both ways: if A scratches B and B does not scratch A, A is harder.
- Never scratch-test minerals that are toxic as dust, such as galena, erythrite or skutterudite, without washing your hands and the surface afterwards.
Hardness of Moroccan minerals
Most collector minerals from Morocco are soft. Almost everything from the lead and cobalt districts sits between 1.5 and 4, softer than a steel knife and often softer than a copper coin. That is why their crystal tips chip so easily and why they must never be packed loose.
| Mineral | Mohs | What it means in practice |
|---|---|---|
| Erythrite | 1.5 to 2.5 | Blades bend and break at a touch |
| Desert rose (gypsum) | 2 | Scratched by a fingernail |
| Galena | 2.5 to 2.75 | Soft and very heavy; cleaves into cubes |
| Wulfenite | 2.5 to 3 | Thin plates snap easily |
| Anglesite | 2.5 to 3 | Brittle; protect edges |
| Calcite | 3 | The scale's own reference for 3 |
| Barite | 3 to 3.5 | Blades split along cleavage |
| Cerussite | 3 to 3.5 | Twins break at the arms |
| Vanadinite | 3 to 4 | Crystal edges chip easily |
| Azurite, malachite, aragonite | 3.5 to 4 | Softer than glass |
| Fluorite | 4 | The scale's own reference for 4 |
| Skutterudite | 5.5 to 6 | Hard for a metallic mineral |
| Amethyst (quartz) | 7 | Scratches glass and steel |
What the scale does not tell you
Hardness is resistance to scratching, not toughness. Diamond is the hardest natural mineral but can still split along its cleavage with one sharp knock. Fluorite (4) and calcite (3) both cleave cleanly in several directions, so a specimen that resists a scratch can still break in a fall. Rocks such as granite or marble are mixtures of minerals, so a single Mohs number for them is only an average.
Questions collectors ask
What is the highest number on the Mohs scale?
10, defined by diamond. It was the hardest known natural mineral when Mohs designed the scale in 1812.
Is 7 on the Mohs scale good?
7 is the hardness of quartz. A mineral of 7 scratches window glass and steel and resists everyday scratching well. Most Moroccan collector minerals are far softer, between 2 and 4.
Why is diamond so much harder than corundum if they are only one step apart?
Because the scale is ordinal. In absolute terms diamond is about four times as hard as corundum, while corundum is about twice as hard as topaz.
How can I test hardness at home?
Use a fingernail (about 2.5), copper (about 3), glass or a steel knife (about 5.5) and a piece of quartz (7). Test on a hidden spot and wipe away powder before judging the mark.
Where are sapphire and ruby on the Mohs scale?
Both are corundum, which defines 9. Only moissanite (9.5) and diamond (10) are harder among popular gemstones.
Sources
- Wikipedia, Mohs scale (reference minerals, absolute hardness, history)
- Wikipédia, Échelle de Mohs (everyday reference materials)
- Terra Specimen mineral guides (hardness of each species)
- Wikipedia, Moissanite (hardness)
- Wikipedia, Beryl (hardness)
- Wikipedia, Garnet (hardness)
- Wikipedia, Jadeite (hardness)
- Wikipedia, Opal (hardness)
- Wikipedia, Turquoise (hardness)
- Wikipedia, Pearl (hardness)