Tool

How hard is that stone?

Every common gem ranked by scratch resistance, and the difference between hardness and toughness that costs people stones.

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The ten reference minerals are the scale itself — Friedrich Mohs picked them in 1812 because each one scratches the one below it. Everything else is placed by comparison against them.

The line that matters

Household dust is largely quartz, which sits at 7. Anything softer than that is being sanded a little every time it is wiped, which is why a softer stone in a ring goes dull long before it goes anywhere near a chip.

Ranked hardest to softest

Diamondreference
10
Hardest known natural material, and still cleavable if struck on the wrong plane.
Moissanite
9.25–9.5
Corundumreference
9
Ruby and sapphire are both corundum — the only difference is colour.
Chrysoberyl
8.5
Topazreference
8
Spinel
8
Emerald
7.5–8
Hard enough, but usually full of inclusions, which makes it fragile in a way hardness does not capture.
Aquamarine
7.5–8
Tourmaline
7–7.5
Quartzreference
7
The line that matters: household dust is largely quartz, so anything below 7 dulls with ordinary wear.
Amethyst
7
Garnet
6.5–7.5
Peridot
6.5–7
Tanzanite
6–7
Soft and cleavable — a poor choice for a ring worn daily.
Moonstone
6–6.5
Opal
5.5–6.5
Also holds water, so it can craze in dry heat.
Orthoclase feldsparreference
6
Apatitereference
5
Fluoritereference
4
Malachite
3.5–4
Coral
3–4
Pearl
2.5–4.5
Nacre, not a mineral. Loses lustre to perfume and acid before it is ever scratched.
Calcitereference
3
Amber
2–2.5
Scratches on almost anything. Store it apart.
Gypsumreference
2
Talcreference
1
The softest. Marked by a fingernail.

8 of the stones listed here sit at 7 or above, which is the practical threshold for something worn on a hand every day. Below it a stone will dull with ordinary wear rather than with accidents — and the bars above show position on the scale, not how much harder one stone is than another.

Hardness is not toughness, and the two get confused constantly

Hardness is resistance to scratching. Toughness is resistance to breaking, and they are different properties that often run in opposite directions. Diamond tops this scale and will still split cleanly if struck on the wrong plane. Jade is far softer and is famously difficult to break. Emerald is hard enough at 7.5 to 8 but is usually so included that it is fragile in practice.

So a high number does not mean a stone is safe in a ring, and a low one does not mean it will shatter. It means precisely one thing: what will and will not scratch it.

A ranking, not a ruler

Friedrich Mohs set the scale out in 1812 by choosing ten minerals where each scratches the one below it. That is all it measures: what will and will not scratch what. It is ordinal, which means the numbers put minerals in order and nothing more — the intervals between them are not equal and were never meant to be.

The gap that makes this concrete is the top one. Diamond at 10 is several times harder than corundum at 9 in absolute terms, while the step from talc at 1 to gypsum at 2 is negligible by comparison. Any chart that draws the scale as evenly spaced bars implies a proportionality that does not exist, which is why the ranking rather than the spacing is what is shown here.

Seven is the number that matters in practice

Ordinary household dust is largely quartz, which sits at 7. Anything softer is being very lightly sanded every time it is wiped, handled or put in a pocket. That is why softer stones in rings go dull long before they go anywhere near chipping — the damage is thousands of invisible scratches rather than one accident.

This puts a practical line under the stones suited to daily wear on a hand. Diamond, corundum in both its forms, spinel, topaz, tourmaline and quartz sit at or above it. Opal, moonstone, tanzanite, peridot, pearl and amber sit below, and are better suited to earrings, pendants and occasional wear where they meet less friction.

Hardness is not toughness

These are different properties and they frequently run in opposite directions. Hardness resists scratching. Toughness resists breaking. Diamond tops the hardness scale and will still cleave if struck along the wrong plane, which is precisely how diamond cutters have shaped it for centuries. Jade is far softer and famously hard to break — it was used for axe heads.

Emerald is the case that catches people out. At 7.5 to 8 it is hard enough on paper, but emeralds are usually heavily included, and those inclusions are fracture planes. An emerald can survive dust indefinitely and still split on a knock a softer, cleaner stone would shrug off.

So a high number does not mean a stone is safe in a ring, and a low one does not mean it will shatter. Read the number as one specific fact about scratching, and treat setting, wear pattern and inclusions as separate questions.

Common questions

What is the hardest gemstone?

Diamond, at 10 on the Mohs scale — the hardest known natural material. Moissanite follows at 9.25 to 9.5, then corundum, which covers both ruby and sapphire, at 9.

What Mohs hardness is safe for an engagement ring?

Seven or above is the usual practical line, because household dust is largely quartz at 7 and anything softer dulls with ordinary wear. That covers diamond, sapphire, ruby, spinel, topaz and tourmaline. Softer stones such as opal, moonstone and tanzanite suit pieces that meet less friction.

Is the Mohs scale linear?

No. It is ordinal — it ranks minerals without the intervals being equal. Diamond at 10 is several times harder than corundum at 9, while the difference between 1 and 2 is very small.

Can a hard gemstone still break?

Yes. Hardness resists scratching and toughness resists breaking, and they are separate properties. Diamond is the hardest material known and will still cleave if struck on the wrong plane, whereas much softer jade is extremely difficult to break.

What is my gold worth?

Weigh it, pick the karat, and see the value of the metal at the live gold price — then what a buyer is likely to hand over, which is a different number.

What is my silver worth?

Sterling, coin, Britannia and fine silver at the live silver price — including why small silver items are often worth less than the trouble of selling them.

What is platinum or palladium worth?

PT950, PT900 and PD950 at live prices. Both metals are denser than gold, which is why the weight surprises people.