Rock IdentifierThe Mohs Hardness Scale Explained
Key takeaways
- Mohs measures scratch resistance, from talc (1) to diamond (10).
- The scale is relative, not linear — diamond is far harder than the number '10' implies.
- Everyday objects (nail, coin, glass) let you test hardness in the field.
What hardness really means
Hardness is a mineral's resistance to being scratched, and it's one of the most reliable identification tools because it barely changes with color or locality. In 1812 Friedrich Mohs picked ten reference minerals, each able to scratch the ones below it, and the scale has been standard ever since.
The ten reference minerals
From softest to hardest, the Mohs standards are:
- 1 · Talc — crushed by a fingernail
- 2 · Gypsum — scratched by a fingernail
- 3 · Calcite — scratched by a copper coin
- 4 · Fluorite — scratched easily by a knife
- 5 · Apatite — scratched by a knife with effort
- 6 · Orthoclase feldspar — scratches glass
- 7 · Quartz — scratches glass easily
- 8 · Topaz
- 9 · Corundum (ruby, sapphire)
- 10 · Diamond — the hardest natural material
Field tests with everyday objects
You don't need a hardness kit to get close. Common references are a fingernail at about 2.5, a copper coin near 3.5, a steel knife blade or nail around 5.5, and ordinary window glass also about 5.5. Try to scratch the mineral with each, working up from soft, and note where it starts resisting.
Test it right
Always scratch on a fresh, clean surface and wipe away the powder — a soft mineral can leave a mark that looks like a scratch but rubs off. Press firmly but not so hard you chip the specimen. And remember the scale is ordinal: the gap between corundum (9) and diamond (10) is enormous, far bigger than between 1 and 2.
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