What the Mohs scale measures
The Mohs scale measures scratch hardness: a mineral scratches every mineral below it and is scratched by every mineral above it. It is a relative scale, not a linear one. The steps are not equal: diamond (10) is several times harder than corundum (9), while the difference between talc (1) and gypsum (2) is tiny.
The 10 reference minerals
| Hardness | Mineral | Everyday comparison |
|---|---|---|
| 1 | Talc | crumbles under the fingernail, feels soapy |
| 2 | Gypsum | scratched by a fingernail (about 2.5) |
| 3 | Calcite | scratched by a copper coin (about 3.5) |
| 4 | Fluorite | easily scratched by a steel knife |
| 5 | Apatite | scratched by a knife with some effort |
| 6 | Orthoclase (feldspar) | scratches glass (about 5.5) |
| 7 | Quartz | scratches glass and steel easily |
| 8 | Topaz | scratches quartz |
| 9 | Corundum (ruby, sapphire) | scratches topaz |
| 10 | Diamond | scratches every other natural mineral |
Your hardness kit
- Fingernail: about 2.5.
- Copper coin or copper wire: about 3.5.
- Steel knife or nail and window glass: about 5.5.
- Steel file: about 6.5.
- Unglazed porcelain (the streak plate): about 7.
How to do the test, step by step
- Choose a fresh, clean surface of the mineral, not a weathered crust: alteration products are often much softer.
- Hold the sample firmly and press the tester across it with a short, decisive stroke.
- Wipe the mark with your finger. A real scratch is a groove that stays; a soft tester (like a coin) often leaves a trail of metal powder that looks like a scratch but rubs off.
- Test in both directions: try to scratch the tester with the mineral too. If neither scratches the other, they have similar hardness.
- Narrow down step by step: if glass is scratched but the steel file is not, the hardness is between 5.5 and 6.5.
On valuable specimens and cut gems, test only on a hidden spot, or do not test at all: a scratch cannot be undone.
Common mistakes
- Testing aggregates. A granular rock can crumble without any real scratch: grains are pulled out, not cut.
- Forgetting that hardness can vary with direction. Kyanite is the textbook case: about 4.5–5 along the length of the crystal and 6.5–7 across it.
- Confusing hardness with toughness. Diamond is the hardest natural mineral, but it can still split along its cleavage if it is hit hard. Hardness is resistance to scratching, not to breaking.
Why hardness matters for jewelry
Ordinary dust contains tiny grains of quartz (hardness 7). Gems softer than 7, such as opal, turquoise or pearls, slowly lose their polish if worn every day on a ring. Sapphire, ruby (9) and diamond (10) resist much better. This is why hardness is one of the first things gemologists check.
Mohs test in GeoCrystal
GeoCrystal includes a practical guide to the Mohs hardness test with reference minerals and everyday objects, and every mineral datasheet shows its hardness. Combined with the other tests in our guide on how to identify a mineral, it lets you narrow down a sample in a few minutes.
GeoCrystal · Mineralogy
WildLife · Nature & hiking