The Mohs scale, and how to use it

In 1812 the German mineralogist Friedrich Mohs looked for a way to rank minerals without instruments. He settled on the simplest one available: what scratches is harder than what is scratched. He picked ten reference minerals, softest to hardest, and the scale has carried his name ever since.

Two centuries later it is still the highest-yield test in amateur identification. It costs nothing, it takes ten seconds, and it eliminates more candidates than any observation.

The ten degrees

Hardness	Mineral	Marker
1	Talc	crumbles under a fingernail, greasy to the touch
2	Gypsum	scratched by a fingernail
3	Calcite	scratched by a copper coin
4	Fluorite	scratched easily by a knife
5	Apatite	scratched with difficulty by a knife
6	Orthoclase (feldspar)	scratches glass, scratched by a file
7	Quartz	scratches glass decisively, and the file
8	Topaz	scratches quartz
9	Corundum (ruby, sapphire)	scratches topaz
10	Diamond	scratches everything else

Watch for a property the scale does not advertise: it is not linear. In absolute hardness, diamond is roughly four times harder than corundum, though a single degree separates them. The jump from 9 to 10 is larger than the entire journey from 1 to 9. Mohs built a ranking, not a measurement.

The standards you already own

Nobody keeps topaz in a drawer. It hardly matters: everyday objects cover the useful range, the one where 95% of picked-up stones live.

Standard	Hardness	What it settles
Fingernail	2.5	gypsum, talc, selenite, amber
Solid copper coin	3 to 3.5	calcite, limestone, malachite
Steel nail or knife blade	5.5	apatite, obsidian, opal
Shard of glass	5.5	the same threshold, the other way round
Unglazed porcelain tile	6.5	feldspar, pyrite
Hardened steel file	6.5	the same
A quartz you are sure of	7	the most useful threshold of all

That last one deserves a word. Keep a chip of colourless quartz — a clear river pebble will do — in your pocket. It is the single most useful standard there is, because the 7 threshold separates the enormous quartz family and its varieties (amethyst, agate, flint, jasper, chalcedony) from almost everything that resembles them.

Running the test properly

Pick an inconspicuous edge. The mark will stay. On a polished or mounted stone, always test below the girdle or on the base.

Press firmly, over two centimetres, once. A hesitant back-and-forth produces an ambiguous mark.

Rub the mark with a finger, then look. This is the step everyone skips, and it is the one that matters. A soft metal — the nail, the coin — leaves a grey or coppery smear on a harder stone that looks a great deal like a scratch. It wipes off with a finger, or with water. A real scratch is a groove: it stays, and a fingernail can feel it.

Test in both directions. If the glass does not scratch the stone and the stone scratches the glass, you are above 5.5. If neither scratches the other, you are around 5.5. The two trials together are worth far more than either alone.

Three traps that falsify the result

Modern coins. Small "copper" coins — euro cents, and most other low-value coins minted since the 1990s — are copper-plated steel. The plating is about a hundred microns thick. If your test goes through it you are suddenly testing against steel at 5.5 instead of copper at 3.5, and you will conclude that a calcite is a feldspar. Use an old solid-copper coin, or skip that degree.

Weathered surfaces. A stone left out in the weather grows a rind softer than its core. Weathered feldspar will take a fingernail scratch. Chip a corner, or test on a fresh area.

Rocks, on principle. A rock is an assembly of minerals of different hardnesses. Scratching a granite measures nothing: the nail will meet mica at 2.5 or quartz at 7 depending on where it lands. Hardness is a mineral test. On a rock, what needs identifying is the assembly — see the general method.

What hardness rules out, in practice

A few classic pairs, and the degree that separates them:

• Amethyst (7) against purple fluorite (4). A knife scratches fluorite effortlessly and does nothing to amethyst. The commonest confusion in the whole purple end of mineralogy, settled in five seconds.
• Pyrite (6 to 6.5) against gold (2.5 to 3). Gold takes a scratch from a copper coin; pyrite scratches glass. And gold, being malleable, flattens instead of chipping.
• Rose quartz (7) against pink calcite (3). Glass settles it.
• Obsidian (5 to 5.5) against manufactured glass (5.5). Here hardness is not enough: they are identical, which is logical — both are glasses. You have to look at the inclusions and the bubbles.

That last example is the most instructive. Hardness is the test that eliminates the most; it is not the test that concludes. It shortens the list; streak, density and fracture close it.