Mohs Hardness Tester
The softest thing that scratches it, and the hardest thing it scratches. The mineral is between them.
Bracket a mineral's hardness with four references you already own: a fingernail is about 2.5, a copper coin 3.5, a steel knife or window glass 5.5, a steel file 6.5. Find the softest thing that marks the specimen and the hardest thing the specimen marks — the mineral sits between them. That single bracket eliminates more candidates than any other test, and it is free.
Between 3.5 and 5.5
A workable bracket. One more reference would narrow it.
4 · Fluorite · 5 · Apatite
- Malachite3.5–4
Fizzes in acid, and heavy for its size — resin imitations are not.
Streak pale green · SG 3.6–4
- Howlite3.5
White beneath any dye. The usual stand-in for turquoise.
Streak white · SG 2.53–2.59
- Chalcopyrite3.5–4
Brassy but soft — a steel knife cuts it where pyrite resists.
Streak greenish-black · SG 4.1–4.3
- Fluorite4
Perfect octahedral cleavage in four directions, and it usually glows under UV.
Streak white · SG 3–3.25
- Obsidian5–5.5
Amorphous glass — no cleavage, and translucent brown at a thin edge.
Streak white · SG 2.3–2.6
- Turquoise5–6
Resists a knife, where dyed howlite gives way to white powder.
Streak white to pale green · SG 2.6–2.8
- Lapis lazuli5–6
Metallic pyrite flecks embedded in the blue, with white calcite that fizzes.
Streak pale blue · SG 2.7–2.9
- Hematite5–6.5
Red-brown streak whatever the surface colour, and barely magnetic.
Streak red-brown · SG 5.2–5.3
- Opal5.5–6.5
Remarkably light, and its colour play must shift as the stone moves.
Streak white · SG 2.05–2.15
- Magnetite5.5–6.5
Strongly magnetic, and it streaks black rather than red.
Streak black · SG 5.1–5.2
The bracket is tight but crowded. Take a streak on unglazed porcelain next, then try a drop of vinegar — between them they split this list in half.
Hardness is the first question, and the cheapest one to answer
Mineral identification has a reputation for difficulty that it does not deserve, and the reason people find it hard is that they start with colour. Colour is the least reliable diagnostic property in mineralogy: quartz occurs as amethyst, citrine, rose quartz, smoky quartz, agate, jasper and tiger's eye, all one species with one hardness. Purple, meanwhile, can mean amethyst at 7 or fluorite at 4 — three whole points apart and separated in one second by a knife.
Hardness is different because it is a property of the crystal structure rather than of whatever impurity happened to be in the water. It does not vary with colour, it does not vary with locality, and it can be measured on a kitchen table with things already in the house.
How to run the test without ruining the specimen
Work upwards, not downwards. Start with a fingernail and move to harder references only while nothing has left a mark, so the first real scratch is also the smallest one you will make. Always test on an inconspicuous face — the base, the back, a broken edge — because a scratch is permanent.
Then wipe. This is the step almost everyone skips and it causes most wrong answers: when a soft reference drags across a hard mineral, the reference loses material and leaves a streak of its own dust that looks exactly like a scratch. Clean the spot with a cloth and look again under a lens. If the line disappears, nothing was scratched.
- Test on a hidden face, and only once per reference.
- Work from soft to hard, stopping at the first genuine mark.
- Wipe the powder away before deciding — dust reads as a scratch.
- Confirm under a hand lens, at an angle, not flat on.
- Use a genuinely copper coin. Much small change is copper-plated steel.
Why a bracket beats a number
A single scratch tells you almost nothing on its own. Knowing that a steel knife marks a stone puts it below about 5.5, which still leaves several hundred minerals. Knowing that the stone also marks a copper coin puts it above 3.5, and the pair of answers between them is a bracket a few points wide with a short list inside it.
That is the whole method, and it is why the tool asks two questions rather than one. A bracket of one point or less has usually done most of the work; a bracket of three points has barely started, and the fix is to add a reference in the middle rather than to test harder.
What hardness cannot do
It cannot separate two minerals of the same hardness, and there are plenty — quartz and amethyst are the same mineral, and agate is too. For those, hardness confirms the family and something else decides the variety.
It also cannot be run on a finished object without damaging it. A polished cabochon, a carved figure or a set stone should not be scratched anywhere a scratch will show, which sometimes means the test is unavailable and the honest answer is that a specific gravity measurement or a jeweller's opinion is the route instead.
And it says nothing about treatment. A dyed howlite bead is exactly as hard as an undyed one, so hardness will tell you the material is not turquoise and will not tell you what has been done to it.
| Mohs | Reference mineral | Household equivalent |
|---|---|---|
| 1 | Talc | Marked by anything; feels greasy |
| 2 | Gypsum (selenite) | A fingernail scratches it |
| 2.5 | — | Fingernail · also amber, and gold |
| 3 | Calcite | A copper coin just marks it |
| 3.5 | — | Copper coin · also malachite, howlite |
| 4 | Fluorite | A steel knife cuts it easily |
| 5 | Apatite | A knife still bites |
| 5.5 | — | Steel knife and window glass · obsidian sits here |
| 6 | Orthoclase feldspar | Scratches glass; a file marks it |
| 6.5 | — | Steel file · pyrite, jade, nephrite |
| 7 | Quartz | Scratches glass and steel; nothing domestic marks it |
| 8 | Topaz | Scratches quartz |
| 9 | Corundum (ruby, sapphire) | Scratches everything but diamond |
| 10 | Diamond | Scratches everything |
The scale is ordinal, not linear — diamond at 10 is many times harder than corundum at 9, not ten percent harder than 9.
Questions we are asked
- How do I test a mineral's hardness at home?
- Use four references you already own. A fingernail is about 2.5, a genuinely copper coin 3.5, a steel knife or window glass 5.5, and a steel file 6.5. Work upwards until something leaves a real mark, then drag the specimen across each reference to see what it marks. The mineral's hardness sits between the two answers.
- Why does my scratch disappear when I wipe it?
- Because it was never a scratch. When a softer reference drags across a harder mineral, the reference loses material and leaves a line of its own powder that looks identical to a scratch. Always clean the spot and re-examine under a lens before concluding anything.
- Is the Mohs scale linear?
- No, it is ordinal — it ranks minerals by which scratches which and says nothing about how much harder each step is. The gap between corundum at 9 and diamond at 10 is far larger than the gap between 8 and 9.
- Can I test hardness on a polished stone or a set ring?
- Not without leaving a permanent mark. On finished pieces, measure specific gravity instead — it is non-destructive and needs only a kitchen scale and a cup of water — or take the piece to a jeweller, who can test it without scratching anything visible.
- Will hardness tell me if a stone has been dyed?
- No. Dye does not change hardness, so a dyed howlite bead is exactly as soft as an undyed one. What hardness does is tell you the material is howlite at 3.5 rather than turquoise at 5–6, which is usually the question that matters.
Before You TestRead the stone itself
Hardness narrows it to a shortlist. Photograph the specimen and the instrument offers a name to start from — then hands you back to the coin and the tile, which are what settle it.