FIELD The Mohs hardness scale
Hardness is one of the fastest, most reliable clues to a mineral’s identity — and you can test it with a fingernail, a coin and a pocket knife.
Introduced by the German mineralogist Friedrich Mohs in 1812, the Mohs scale ranks minerals from 1 to 10 by their resistance to scratching. It is one of the oldest diagnostic tests in geology and still one of the most useful in the field, because the answer costs nothing and takes seconds.
What the Mohs scale actually measures
The scale measures scratch hardness: whether one material can leave a permanent groove in another. The rule is simple — a harder mineral scratches a softer one, never the reverse. If mineral A scratches mineral B but B cannot scratch A, then A is the harder of the two. That is the whole logic behind every field test you will run.
Crucially, the Mohs scale is ordinal, not linear. It ranks minerals in order but the steps are uneven. Diamond, at 10, is roughly four times harder in absolute terms than corundum at 9, even though they sit only one step apart. So treat the numbers as a ladder of rank, not a set of measured quantities.
The ten reference minerals
Mohs chose ten common minerals, each of which scratches everything below it:
Soft (1–5)
- 1 – Talc: crumbles under a fingernail
- 2 – Gypsum: a fingernail (~2.5) scratches it
- 3 – Calcite: a copper coin scratches it
- 4 – Fluorite: easily cut by a knife
- 5 – Apatite: a steel blade just scratches it
Hard (6–10)
- 6 – Orthoclase feldspar: scratches glass
- 7 – Quartz: scratches steel and glass
- 8 – Topaz
- 9 – Corundum (ruby, sapphire)
- 10 – Diamond: scratches everything
Field tools you already carry
You do not need a lab kit. A handful of everyday objects have well-known hardness values, and together they bracket most of the scale:
- Fingernail — about 2.5. Scratches gypsum and talc; will not touch calcite.
- Copper coin — about 3.5. (A pre-1982 US penny is genuinely copper; newer ones are zinc and softer.)
- Steel knife blade or nail — about 5.5. The single most useful tester in the field.
- Glass plate — about 5.5. A mineral that scratches glass is 6 or harder.
- Streak plate (unglazed porcelain) — about 7, and doubles as a streak-colour test.
How to run a scratch test properly
Do the test on a fresh, clean surface, not a weathered rind, and press firmly with a single decisive stroke. Then wipe and look closely: a real scratch is a groove you can feel and that stays after rubbing. A soft tester dragged across a hard mineral often leaves a metallic smear — deposited steel or copper — that wipes clean and fools beginners into thinking they made a scratch. Use a hand lens to be sure.
Test on an inconspicuous spot or a broken chip, because the scratch is permanent. Avoid running the test along cleavage planes or across grain boundaries, where a mineral can appear softer than it truly is, and repeat the test in both directions to confirm which mineral actually cut which.
Why hardness helps you identify rocks
Hardness narrows the field quickly. A translucent pebble that scratches glass is almost certainly a member of the quartz family — agate, jasper, chalcedony or chert — and not a soft look-alike such as calcite or barite. A crystal that a knife cuts easily is not quartz, no matter how “crystalline” it looks. Pair a hardness reading with colour, lustre, streak and crystal habit and you can usually name a common mineral without any instruments.
It is also a fast fake-detector. Glass imitations of gemstones sit around 5.5 and will not scratch quartz, while the genuine stone often will. Hardness is only one line of evidence, but it is a decisive one.
What the scale does not tell you
Hardness is not the same as toughness. Diamond is the hardest natural material yet it is brittle and can be cleaved with a sharp tap; jade, by contrast, is only moderately hard but extraordinarily tough because of its interlocking fibres. Do not confuse “hard to scratch” with “hard to break.”
A few minerals are also variable or directional. Kyanite famously reads about 4.5 along its length but 6.5 across it. Weathering, impurities and fine intergrowths can shift a reading too, so use hardness as one clue among several rather than a verdict on its own.
Frequently asked questions
Who invented the Mohs scale?
The German geologist and mineralogist Friedrich Mohs devised it in 1812 by ordering minerals according to which could scratch which. The same ten reference minerals are still used today.
Is a higher Mohs number proportionally harder?
No. The scale is ordinal — it only ranks order. The gaps are uneven, and diamond (10) is far more than one step harder than corundum (9) in absolute terms.
What household items can I use to test hardness?
A fingernail (~2.5), a copper coin (~3.5), a steel knife or nail (~5.5) and a glass plate (~5.5). Together they let you place most minerals within a step or two.
Does scratching glass mean I’ve found quartz?
It means the mineral is 6 or harder, which includes quartz but also feldspar, topaz and others. Combine the result with translucency, lustre and pattern — see our guide to agate vs. jasper vs. chert.
Can I damage a specimen by testing it?
Yes — a scratch is permanent. Test on a broken chip or an inconspicuous back surface, and never scratch-test a finished gemstone or a display-quality crystal.
Want reference minerals to practise on?
Sponsored — partner shop · Minerals Kingdom
New to the hobby? Start with how to start rockhounding, then build a small hardness kit and practise on stones you have already identified. Hardness pairs naturally with weighing a specimen for its specific gravity, another instant field test.
Written by The Field & Stone Editors · Published by KEVALEX Group.