Mohs Hardness Scale Table
| Rank | Mineral | Field anchor | The test |
|---|---|---|---|
| 1 | Talc | smears like pencil lead | fingernail gouges it |
| 2 | Gypsum | fingernail (2.5) wins | nail marks it easily |
| 3 | Calcite | copper coin (3.5) wins | penny scratches it |
| 4 | Fluorite | steel nail (4.5) wins | nail scratches it |
| 5 | Apatite | knife just wins (5.5) | blade barely marks it |
| 6 | Feldspar | steel file (6.5) wins | scratches glass |
| 7 | Quartz | scratches glass and file | sand marks windows |
| 8 | Topaz | scratches quartz | only 9-10 scratch it |
| 9 | Corundum | ruby, sapphire | only diamond wins |
| 10 | Diamond | scratches everything | only diamond marks it |
The Mohs scale ranks scratch resistance from 1 to 10: a harder material scratches a softer one, never the reverse. Friedrich Mohs picked ten minerals for the ladder in 1812 - talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum, diamond - and the scale survives because the test needs no lab: scratch the mineral against the anchor objects in the table and read your position from what marks what.
Type two hardness values into the comparator and it tells you which one scratches which, and what the nearest ladder minerals are. Two warnings from the field guide: the scale is ordinal, so diamond at 10 is not 'one step better' than corundum at 9 - on absolute scales diamond is roughly four times harder - and hardness is not toughness, which is why a diamond ring survives scratches for a lifetime but still chips when it is knocked against a counter.
How to use
- Type two hardness values (or use the everyday anchors: fingernail 2.5, copper coin 3.5, knife blade 5.5, window glass 5.5, steel file 6.5).
- The verdict shows which scratches which; at equal hardness neither cuts the other cleanly, they just wear each other.
- Test in an inconspicuous spot - a scratch test is a destructive test, and the winner and loser both carry the mark.
Frequently asked questions
What is the hardest natural material?
Diamond, alone at 10 on the Mohs scale - nothing natural scratches it except another diamond. Lab-made contenders such as lonsdaleite and wurtzite boron nitride are calculated to be harder in specific crystal directions, but they are not the benchmarks jewelers and mineralogists test against; for every practical purpose the top of the ladder is diamond.
Is the Mohs scale linear?
No - it is an ordinal ranking, not a measurement. Each step just means 'scratches everything below it'. On absolute hardness scales the gaps are enormous: quartz at 7 is roughly one hundred times harder than talc at 1, and diamond at 10 is about four times harder than corundum at 9. That is why a mineral one step up can feel dramatically tougher in the hand.
Can anything scratch a diamond?
Only another diamond. This is why diamonds are graded with diamond dust and why loose stones are stored separately - two diamonds in one pouch will mark each other. Hardness is not toughness, though: diamond cleaves along crystal planes, so a knock at the right angle can chip a ring that no material could scratch.
What hardness scratches glass?
Anything above about 5.5 - window glass sits at 5.5 on the scale, so a steel file (6.5), quartz (7, which is why sand scratches glass), topaz and corundum all mark it, while a copper coin (3.5) and a steel knife blade (right at 5.5) generally do not. Glass cutters work precisely because carbide and diamond sit far above the glass they score.
Where do common objects sit on the scale?
A fingernail scratches at about 2.5 (it marks gypsum but not calcite), a copper penny about 3.5, a pocketknife blade about 5.5, window glass 5.5, and a steel file 6.5. Collectors use exactly these objects as a field kit: if your specimen is scratched by a fingernail it is below 3, if it scratches glass it is above 5.5, and if it scratches a file it is quartz-class or better.