The Mohs scale ranks minerals by scratch resistance on a scale of 1 to 10. Talc sits at 1, diamond at 10, and each mineral on the list can scratch the ones below it. It is the hardness figure quoted on nearly every gem profile, and it tells you a lot about how a stone will hold its polish. It does not tell you how much harder one mineral is than another, or how well a stone survives a hard knock.
Where the scale came from
The German mineralogist Friedrich Mohs devised the scale in 1812. He wanted a quick field method for telling minerals apart, and scratch testing needed no lab equipment. Mohs picked ten common, easy-to-find minerals and put them in order by which scratched which. The idea of ranking by scratching was older than Mohs, but his set of reference minerals is the one that stuck. Geologists and gemologists still use it two centuries later.
The ten reference minerals
| Mohs number | Reference mineral | Gem connection |
|---|---|---|
| 1 | Talc | Soapstone carvings |
| 2 | Gypsum | Alabaster and satin spar |
| 3 | Calcite | Marble, the host rock for some ruby |
| 4 | Fluorite | Faceted for collectors |
| 5 | Apatite | Neon blue apatite |
| 6 | Orthoclase feldspar | Moonstone family |
| 7 | Quartz | Amethyst, citrine, agate |
| 8 | Topaz | Imperial and blue topaz |
| 9 | Corundum | Ruby and sapphire |
| 10 | Diamond | Diamond |
Several of these are gems in their own right. Apatite marks the 5 position, topaz marks 8, and corundum, the mineral behind ruby and sapphire, marks 9.
A few household objects fill in the gaps for rough testing. A fingernail is about 2.5, a copper coin about 3, window glass and a steel knife blade about 5.5, and a hardened steel file about 6.5. These are approximations, since the actual hardness of glass and steel varies with composition.
Why the steps are uneven
The Mohs scale is ordinal. It puts minerals in order, but the gaps between numbers are not equal. Moving from 1 to 2 is a small step. Moving from 9 to 10 is enormous.
Instruments that measure hardness on a continuous scale make this plain. Older sclerometer tests, which measure the force needed to scratch a surface, give figures commonly cited like these (approximate, with quartz set near 100):
| Mineral | Mohs | Approximate absolute hardness |
|---|---|---|
| Talc | 1 | 1 |
| Gypsum | 2 | 2 |
| Calcite | 3 | 14 |
| Fluorite | 4 | 21 |
| Apatite | 5 | 48 |
| Orthoclase | 6 | 72 |
| Quartz | 7 | 100 |
| Topaz | 8 | 200 |
| Corundum | 9 | 400 |
| Diamond | 10 | 1500 |
The exact numbers vary from source to source and with the test method, but the shape of the curve does not. Diamond is several times harder than corundum, and the gap between them is larger than the gap between talc and corundum. Modern labs use indentation tests to get these continuous values, a method covered on the Vickers hardness page.
Hardness can also vary by direction within a single crystal. Kyanite is the classic case: it measures about 4.5 along the length of its blades and around 6.5 to 7 across them. Diamond also shows small directional differences, which is why diamond powder can polish diamond.
Why quartz at 7 matters
Ordinary household dust contains tiny grains of quartz and other silicate minerals. Quartz sits at 7, so any gem softer than 7 will slowly pick up fine scratches from dust, cleaning cloths and everyday contact. Facet edges round off and polished surfaces go hazy over years of wear.
This is why 7 works as a rough dividing line in jewelry. Stones at 7 and above, such as quartz itself, beryl, topaz, spinel and corundum, keep a crisp polish for a long time. Stones below 7, such as opal, turquoise, moonstone and pearl, need more gentle handling and the occasional repolish. It also means you should store gems separately: a sapphire tossed in a pouch with a peridot will scratch the peridot.
Hardness of common gems
| Gem | Mohs hardness |
|---|---|
| Diamond | 10 |
| Synthetic moissanite | 9.25 |
| Ruby and sapphire | 9 |
| Chrysoberyl and alexandrite | 8.5 |
| Cubic zirconia | 8-8.5 |
| Spinel | 8 |
| Topaz | 8 |
| Emerald and aquamarine (beryl) | 7.5-8 |
| Tourmaline | 7-7.5 |
| Quartz (amethyst, citrine) | 7 |
| Garnet | 6.5-7.5 |
| Zircon | 6.5-7.5 |
| Peridot | 6.5-7 |
| Jadeite | 6.5-7 |
| Tanzanite | 6-7 |
| Nephrite | 6-6.5 |
| Moonstone | 6-6.5 |
| Opal | 5.5-6.5 |
| Turquoise | 5-6 |
| Lapis lazuli | 5-5.5 |
| Malachite | 3.5-4 |
| Pearl | 2.5-4.5 |
| Amber | 2-2.5 |
Ranges reflect natural variation in composition and the fact that published sources round differently. The figures for synthetic moissanite and cubic zirconia explain why both make durable diamond look-alikes.
How hardness is tested
In a classroom or field setting, you test hardness by trying to scratch an unknown mineral with a reference mineral or a tool of known hardness. If the reference scratches the sample, the sample is softer. Hardness pencils with pointed mineral tips make this tidier. A real scratch leaves a groove, while a softer mineral rubbed on a harder one leaves a powder trail that wipes away, so testers check with a loupe.
Scratch testing damages the surface, so it has no place on a cut gem. Gemologists identify finished stones with optical and physical tests instead: refractive index, specific gravity, the spectroscope and the microscope. The instruments page covers those tools. Hardness testing is limited to rough material, and even then a tester picks an unseen spot.
Hardness is not toughness
Hardness is resistance to scratching. Toughness is resistance to breaking, chipping and cracking. The two are separate properties, and confusing them is one of the most common mistakes buyers make.
Diamond is the hardest natural material, yet it can split with a sharp blow because it has perfect cleavage in four directions. The cleavage page explains how that works. Jade sits at only 6 to 7, yet both jadeite and nephrite are among the toughest gem materials known. Their interlocking mass of tiny crystals or fibers resists fracture, which is why ancient cultures could carve jade into tools and thin vessels.
A full picture of how a stone will wear combines hardness, toughness and stability against heat, light and chemicals. The gemstone durability guide puts those factors together for the most common jewelry stones.
Frequently asked questions
Is a Mohs 9 gem almost as hard as diamond?
No. The scale is ordinal, and the gap between corundum at 9 and diamond at 10 is very large. Diamond is several times harder than corundum by absolute measurements.
What Mohs hardness is good for an everyday ring?
Stones at 7 or above resist the quartz grains in dust and hold their polish well. Toughness and cleavage matter too, so a hard stone with easy cleavage, such as topaz, still needs a protective setting.
Can I test a gem's hardness at home?
It is best not to. Scratch tests leave permanent marks, and they cannot reliably identify a cut stone. A gemologist can identify the stone without damaging it.
Why do some gems have a hardness range instead of one number?
Natural variation in chemistry changes hardness slightly, and some crystals are harder in one direction than another. Sources also round values differently.