Hardness is one of the most useful clues in identifying a mineral, and one of the few you can measure properly at home. You don't need a lab or a special kit. A fingernail, a coin, a steel knife and a piece of glass will get you surprisingly far, as long as you know how to read the results.
What the Mohs scale measures
In 1812 the German mineralogist Friedrich Mohs put ten familiar minerals in order of scratch resistance. Each mineral on the list can scratch the ones below it and is scratched by the ones above it. That's still the scale geologists use in the field today.
Hardness in this sense means resistance to being scratched. It is not the same as toughness. Diamond is the hardest natural mineral, yet it can chip or split along its cleavage if struck the wrong way. Jade, by contrast, is only moderately hard but very tough because of its interlocking fibrous structure.
| Hardness | Mineral | Simple check |
|---|---|---|
| 1 | Talc | Feels greasy; easily scratched by a fingernail |
| 2 | Gypsum | Scratched by a fingernail |
| 3 | Calcite | Scratched by a copper coin, not by a fingernail |
| 4 | Fluorite | Scratched easily by a steel knife |
| 5 | Apatite | Scratched by a knife with some effort |
| 6 | Orthoclase feldspar | Scratches glass; scratched by a steel file |
| 7 | Quartz | Scratches glass and steel easily |
| 8 | Topaz | Scratches quartz |
| 9 | Corundum (ruby, sapphire) | Scratches topaz |
| 10 | Diamond | Scratches everything else |
The scale is ordinal, not even
The numbers show order, not equal steps. The gap between 9 (corundum) and 10 (diamond) is far bigger than the gap between 1 and 2. Diamond is several times harder than corundum when measured with laboratory instruments. For identification this doesn't matter much, but it explains why "hardness 9" gemstones still lose easily to diamond tools.
Your household hardness kit
These everyday items fill in the gaps between the reference minerals. The values are approximate, because metals and glass vary with their composition.
- Fingernail: about 2.5
- Copper coin: about 3 to 3.5. Many modern coins are plated steel or zinc, so results vary; an older solid copper coin is the most reliable.
- Steel knife blade or iron nail: about 5 to 5.5
- Glass (a plate or bottle): about 5.5
- Steel file or hardened tool steel: about 6.5
- Unglazed porcelain tile (streak plate): about 6.5 to 7
- A piece of quartz: 7
With just these you can sort most minerals into useful bands: softer than a fingernail, between fingernail and coin, between coin and glass, and harder than glass. The last split, whether a mineral scratches glass or not, is one of the most frequently used tests in any identification key.
How to do a scratch test properly
- Pick a fresh, clean, inconspicuous spot. Weathered surfaces are often softer than the fresh mineral underneath. Choose an edge or the underside so a scratch won't spoil the specimen.
- Hold the specimen firmly on a table. If you're using glass, lay the glass flat on the table rather than holding it in your hand, and tape any sharp edges.
- Press the point of the testing tool into the surface and drag it a centimetre or so with firm, steady pressure. Don't hammer or saw back and forth.
- Wipe the spot with a damp finger and look at it with a magnifier.
- Test both ways. If the tool scratches the stone, try the stone on the tool too. Two materials of similar hardness can scratch each other slightly.
Scratch or streak? Don't fool yourself
Most mistakes in hardness testing come from mistaking a mark for a scratch. Before you record a result, check which of these you're looking at:
- A real scratch is a groove you can feel with a fingernail. It doesn't rub off, and under a lens it has sharp edges.
- A metal streak happens when a soft tool, such as a coin, rubs off on a harder stone. It looks like a grey or coppery line and wipes away. The stone scratched the coin, not the other way round.
- A powder line happens when a soft mineral leaves its own powder on a harder surface, the same thing that happens in a streak test. A chalky white line on glass usually means your mineral is softer than glass. Rub it off and check for a groove.
- Crumbling grains can make a rock seem soft. In a coarse sandstone, for example, a knife may knock individual quartz grains loose without scratching them, because the cement holding them is weak. Always test one mineral grain, not the rock as a whole.
If you're unsure, repeat the test on a different spot. Consistent results are what count.
Test the mineral, not the rock
A rock is a mix of minerals, often of different hardnesses. Granite contains quartz (7), feldspar (6) and mica (about 2.5 to 3), so it can give you three different answers depending on where you scratch. Pick out a single grain with a lens and test that. If you're still working out what kind of stone you have, start with how to identify a rock.
Some minerals have more than one hardness
A few minerals are noticeably harder in one direction than another. The classic example is kyanite, which can be scratched by a knife along the length of its blade-shaped crystals but not across them. Calcite also shows small differences on different faces. This is uncommon, but it's worth knowing if your results seem inconsistent.
Common identification puzzles that hardness solves
- Quartz vs calcite: quartz scratches glass, calcite can't be scratched by a fingernail but is easily scratched by a knife. See quartz, calcite or glass?
- Pyrite vs gold: pyrite is about 6 to 6.5 and brittle. Gold is about 2.5 to 3 and so soft that a pin will dent it.
- Fluorite vs amethyst: both can be purple; fluorite is 4 and quartz is 7.
- Glass imitations vs gemstones: most glass is about 5.5, so many imitations are softer than the gem they're imitating. See real vs fake crystals.
When not to scratch
A scratch test is permanent damage. Skip it, or leave it to a professional, when:
- The stone is a cut gemstone, a piece of jewellery or a polished item.
- It's a fine crystal specimen where any mark would lower its appeal. See is my rock valuable?
- It might be a meteorite, which should be kept intact for verification.
- The mineral may be hazardous to powder. Some minerals contain toxic elements, and some fibrous minerals should not be scraped or sanded. Work on a small area and wash your hands afterwards.
Tip: keep a tiny labelled kit in a box: an old copper coin, a steel nail, a small glass square with taped edges, an unglazed tile and a quartz pebble. With those five items you can bracket the hardness of most minerals you'll find.
Hardness is one clue, not the answer
A hardness reading narrows things down, but many unrelated minerals share the same number. Combine it with streak, luster, cleavage, density and crystal shape for a confident answer. A photo app such as AI Stone lists the expected Mohs hardness for a suggested match, with a plain-language note on what that means in practice, but a photo can't measure hardness. Your scratch test is how you check whether the suggestion fits the stone in your hand.