Mohs hardness scale, with a scratch test you can actually do

Friedrich Mohs published his scale in 1812 and it has survived unchanged for one reason: it needs nothing. Ten reference minerals, arranged so that each scratches the one below it, and a field geologist with a knife and a coin can place any specimen inside it in seconds.

What it measures is scratch resistance — resistance to having the surface bonds broken by a point drawn across it. It is not toughness, not strength, and not durability, and conflating those is where almost every misunderstanding of the scale begins.

It is also ordinal, not proportional. The intervals are not equal and were never intended to be. Measured on an absolute indentation scale, corundum at 9 is roughly four times as hard as topaz at 8, and diamond at 10 is roughly four times corundum again. The gap between 9 and 10 is larger than the gap between 1 and 9. That single fact explains why diamond is in a category of its own rather than one notch above ruby.

1. Pick an inconspicuous spot on a fresh surface. Weathered faces are softer than the interior of the same mineral, and a scratch across the display face of a good specimen is permanent.
2. Press firmly and draw once, a short stroke of a few millimetres. Repeated scrubbing produces a groove even in hard material and tells you nothing.
3. Rub the mark with your thumb, then look. This is the step everyone skips. A steel blade drawn across quartz leaves a grey line that looks exactly like a scratch and wipes straight off — that is metal deposited on the mineral, and it means the mineral is harder than the steel, the opposite of what it looks like.
4. Confirm with a hand lens. A real scratch is a continuous groove that catches light along its length.
5. Work upward. Start with a fingernail and move up through coin, knife, glass and file until something bites. That brackets the hardness between two values, which is all a scratch test can ever give you.

Two properties will mislead you. Cleavage can make a mineral flake away under a point, which looks like a soft scratch and is not — a nail drawn across mica or gypsum along the cleavage plane peels a sheet off. And hardness can be directional: kyanite is about 4.5 measured along the length of its blades and about 7 measured across them, which is diagnostic for the species and which will look like a contradictory result if you do not know to expect it.

Diamond is the hardest natural substance and it will shatter if you hit it with a hammer on the right plane. Jade is only 6 to 7 and it is so tough that it was used for axe heads and adzes for thousands of years. These are different properties and the confusion has practical consequences.

Two consequences worth carrying around. Topaz is an 8 and has perfect basal cleavage, so a topaz ring survives everyday abrasion beautifully and can split from one sharp knock on a doorframe. Opal is only 5.5 to 6.5 and, more importantly, holds water in its structure, so it crazes with dry heat regardless of how carefully it is protected from scratching.

The single most useful number in the whole scale is 7, because ordinary household dust contains quartz. Anything softer than 7 is being sanded by the air it sits in, slowly, forever.

This is why the practical guidance for a stone worn every day — an engagement ring, in particular — is to stay at or above 7, and preferably above 8. Diamond at 10, sapphire and ruby at 9, spinel at 8, and topaz at 8 with the cleavage caveat all qualify. Emerald is nominally 7.5 to 8 but is usually heavily included and frequently oil-treated, which makes it far less durable than the number suggests. Tanzanite at 6 to 7 and opal at 5.5 to 6.5 are genuinely unsuited to daily wear on a hand and are far better in earrings or a pendant, where nothing hits them.

The same logic governs stone countertops. Granite is dominated by quartz and feldspar at 6 to 7 and shrugs off knives; marble is calcite at 3 and both scratches and etches on contact with lemon juice or vinegar, which is a chemical reaction rather than an abrasion and cannot be prevented by sealing. Quartzite is a different rock again and is harder than granite. The trade names in a showroom are unreliable, and a scratch test on a sample offcut settles it in ten seconds.

Hardness is a useful screening tool for gem identification and a poor final answer. Two rules keep it honest.

Never scratch a faceted stone. Test on the girdle — the thin unpolished band around the widest part — or better, do not scratch it at all. A visible scratch on a table facet destroys value.

Hardness cannot distinguish a diamond from its two best simulants. Moissanite is 9.25 and cubic zirconia is 8 to 8.5; both scratch glass effortlessly, so the classic "it scratches glass so it must be a diamond" test proves nothing at all. The properties that do separate them are thermal conductivity, which is what a jeweller's diamond tester measures, and double refraction, which moissanite shows and diamond does not — look through the table at the opposite facet edges under magnification and moissanite doubles them.

The reverse test is more useful than the forward one: anything a steel knife scratches easily is not a diamond, a sapphire, a ruby or a topaz. That eliminates a great deal quickly. For anything that survives it and matters financially, the answer is a gemmological laboratory rather than a kitchen table.