How the Mohs Scale Ranks Gemstone Hardness
The Mohs scale ranks minerals from 1 to 10 by one property: how well they resist scratching.
Jewelers lean on it because scratch resistance decides how a gemstone looks after years of wear. The Mohs hardness scale is the right starting point for choosing and caring for a gem, but it measures only one of the three factors that decide how a stone holds up.
Key Takeaways
- The Mohs scale measures scratch resistance only. A higher number scratches a lower one, and that is all the number tells you.
- The steps are uneven. Diamond (10) is roughly four times harder than sapphire and ruby (9), while sapphire is only about twice as hard as topaz (8).
- Quartz sits at 7, and quartz is a major part of household dust. Gems softer than 7 slowly lose their polish to everyday dust.
- Hardness is not durability. Toughness (resistance to chipping and breaking) and stability (resistance to heat, light, and chemicals) matter just as much.
- A ring takes far more abuse than a pendant or earrings, so gemstone hardness should weigh more heavily when choosing a ring stone.
What Is the Mohs Scale?
The Mohs scale is a ranking of ten reference minerals, each able to scratch every mineral below it. Any other material is placed on the scale by testing what it can scratch and what scratches it.
The ten reference minerals, from softest to hardest:
| Mohs Hardness | Reference Mineral | Everyday Comparison |
|---|---|---|
| 1 | Talc | A fingernail easily scratches it |
| 2 | Gypsum | A fingernail (about 2.5) scratches it |
| 3 | Calcite | A copper coin (about 3 to 3.5) scratches it |
| 4 | Fluorite | Scratches a copper coin; a steel knife scratches it |
| 5 | Apatite | A steel knife blade (about 5.5) scratches it |
| 6 | Orthoclase feldspar | A steel file (about 6.5) scratches it |
| 7 | Quartz | Scratches a steel file and window glass |
| 8 | Topaz | Scratches quartz |
| 9 | Corundum (ruby and sapphire) | Scratches topaz |
| 10 | Diamond | Scratched only by another diamond |
Materials that fall between two reference minerals get a half step. If apatite scratches a sample but fluorite does not, that sample rates about 4.5.
The National Park Service describes the scale as a field tool for identifying minerals with common objects like a fingernail, a penny, or a nail. That simplicity is why the Mohs scale is still in everyday use two centuries after it was introduced.
Mohs Hardness of Popular Gemstones
Gemstone hardness ranges from 10 for diamond down to 2.5 for pearl. The values below come from GIA’s gem care guides, and several are ranges because natural composition varies from stone to stone.
| Gemstone | Mohs Hardness | What It Means in Practice |
|---|---|---|
| Diamond | 10 | Scratch resistant to everything except another diamond |
| Sapphire and ruby | 9 | The hardest colored gems in common use |
| Alexandrite | 8.5 | Hard, with excellent toughness |
| Topaz | 8 | Harder than dust; resists everyday scratches |
| Spinel | 8 | Hard, with good toughness |
| Emerald | 7.5 to 8 | Scratch resistant, but internal fractures make it vulnerable to knocks |
| Aquamarine | 7.5 to 8 | Durable for most jewelry with sensible care |
| Tourmaline | 7 to 7.5 | At or just above dust hardness |
| Amethyst and citrine | 7 | Quartz family; equal to dust, so polish holds but can dull slowly |
| Garnet | 6.5 to 7.5 | Varies by type; most red garnets sit near the top of the range |
| Peridot | 6.5 to 7 | Just under dust hardness; worth protecting in rings |
| Tanzanite | 6 to 7 | Softer and less tough; better suited to earrings and pendants |
| Moonstone | 6 to 6.5 | Soft, with poor toughness |
| Opal | 5 to 6.5 | Soft and sensitive to heat and dryness |
| Turquoise | 5 to 6 | Soft and porous; absorbs oils and cosmetics |
| Pearl | 2.5 | The softest common jewelry gem; scratches easily |
For the full month-by-month lineup, see our official birthstone list.
Why 7 Is the Number That Matters Most
For jewelry, the most useful line on the Mohs scale sits at 7, the hardness of quartz. Sand and dust are made largely of quartz, so any gem softer than 7 can be scratched by the dust that settles on everything.
The damage is gradual. A softer stone does not crack from dust. It slowly loses the crisp, mirror-like polish that makes a cut gem bright, and the surface starts to look hazy or soft-edged.
Dust hardness leads to three practical habits:
- Wipe softer gems with a damp, soft cloth instead of buffing them dry, which drags dust across the surface.
- Store softer pieces in their own pouch or compartment.
- Keep diamonds apart from other gems in a jewelry box, since a diamond will scratch everything it rubs against.
Gems at 7 and above resist dust well. Gems below 7 can still be worn for years, but they reward a little more care.
Where the Mohs Scale Came From
German mineralogist Friedrich Mohs (1773 to 1839) first proposed his hardness scale in 1812 as part of a broader system for identifying minerals by their visible traits. He published the finished version in his Grundriss der Mineralogie between 1822 and 1824.
The gap between the 1812 proposal and the 1822 publication is why some sources date the scale to 1812 and others, including GIA, to 1822. Both dates are correct about different steps.
The underlying idea is much older. Comparing stones by which one scratches the other dates back to the Greek writer Theophrastus around 300 BC.
Why the Mohs Scale Is Not Linear
The Mohs scale is a ranking, not a measurement. The numbers show order, not distance, so a one-step jump near the top of the scale means far more than a one-step jump near the bottom.
Instruments called sclerometers measure absolute scratch hardness. On that measurement, topaz scores about 200, corundum about 400, and diamond about 1,500.
Put in jewelry terms:
- A sapphire is about twice as scratch resistant as a topaz.
- A diamond is nearly four times as scratch resistant as a sapphire.
- The gap between 9 and 10 is larger than the gap between 1 and 9 combined.
The size of the jump from 9 to 10 is why diamond is the only gem in common use that nothing else in a jewelry box can scratch.
Hardness vs. Toughness: Why a High Number Isn’t Enough
Hardness tells you whether a gem will scratch. It says nothing about whether a gem will chip, crack, or break when it hits something. GIA evaluates gem durability on three separate factors: hardness, toughness, and stability.
Toughness: Resistance to Chipping and Breaking
Toughness depends on how a gem’s atoms bond and whether the crystal has weak planes it prefers to split along. A gem can be very hard and still fairly brittle.
Diamond is the clearest example. It tops the Mohs scale, yet a hard blow can cleave or fracture it, and pear and marquise shapes with exposed points are the most prone to chipping.
Jade shows the reverse. On the fracture toughness scale GIA cites, nephrite jade scores around 225,000 while diamond scores roughly 5,000 to 8,000. Jade sits well below diamond on the Mohs scale, but it resists breaking far better.
A few gems where toughness changes the story:
- Emerald rates 7.5 to 8 but has only fair to good toughness because most emeralds contain natural fractures. GIA notes it needs more care in wear than ruby or sapphire.
- Moonstone has two cleavage directions, which is why it is usually set in pendants and earrings rather than rings.
- Alexandrite combines 8.5 hardness with excellent toughness and no cleavage, which makes it a strong choice for a daily-wear ring.
Stability: Resistance to Heat, Light, and Chemicals
Stability covers everything else that can damage a gem: temperature swings, humidity, sunlight, and the chemicals in daily life. None of it shows up on the Mohs scale.
- Opal can develop fine cracks, called crazing, from heat or from drying out. Low-humidity spaces like safes can contribute.
- Pearls are damaged by acids, including those in perfume, hairspray, cosmetics, and perspiration. Pearl jewelry should go on last and come off first.
- Sudden temperature changes can fracture tanzanite, opal, and moonstone.
- Amethyst, citrine, and some topaz can fade with long exposure to strong light.
- Most emeralds are treated with oils or resins to fill fractures. Ultrasonic and steam cleaners can force those fillers out.
What the Mohs Scale Means for the Metal
The metal holding a gem is much softer than the gem itself. Pure gold sits around 2.5 on the Mohs scale, and the gold alloys used in fine jewelry are harder but still well below quartz.
The softness of gold is why prongs wear thin and ring shanks thin out over years of daily contact. GIA notes that bent or broken prongs expose a stone’s edges to chipping, and a diamond loose in its mounting can wear through the metal prongs holding it.
Worn metal is easy to miss until a stone is gone. Having prongs checked on a schedule and repairing worn prongs before a stone shifts protects the gem from getting lost or chipped.
How to Use the Mohs Scale When Choosing Jewelry
Match the gem to how the piece will be worn. Rings collide with doorframes, countertops, and car keys every day, while earrings and pendants rarely touch anything harder than fabric.
Rings and Everyday Pieces
For a ring worn daily, gems at 8 and above with good toughness hold up best: diamond, sapphire, ruby, alexandrite, and spinel. Stones in the 7 to 8 range work well in rings with a little more care, especially in a protective setting.
Earrings, Pendants, and Occasional Wear
Softer or less tough gems like opal, tanzanite, moonstone, turquoise, and pearl are well suited to earrings and pendants. They can go in rings too, but those rings are better saved for occasions than worn through a workday.
Settings That Protect Softer Stones
The setting can make up for some of what a gem lacks. GIA recommends bezels, partial bezels, or V-shaped prongs to protect vulnerable points and corners.
Our guide to diamond and gemstone setting types covers how each style shields a stone. If a softer gem is headed for a ring, a protective setting is worth discussing at the design stage of a custom jewelry piece.
Putting the Mohs Scale to Work
The Mohs scale answers one question well: will this gem scratch? Use 7 as the dividing line, remember that the steps near the top are enormous, and then check toughness and stability before deciding how a stone should be worn. A gem chosen and set with all three in mind will look the way it should for decades.
Frequently Asked Questions
What Is the Hardest Gemstone on the Mohs Scale?
Diamond is the hardest at 10 on the Mohs scale, and only another diamond can scratch it. Ruby and sapphire, both forms of corundum, are next at 9.
Is Anything Harder Than Diamond?
Not among natural gemstones. Diamond sets the top of the Mohs scale at 10. A few lab-made materials, such as aggregated diamond nanorods, have tested harder in research settings, but none are used in jewelry.
Can a Diamond Chip or Break?
Yes. Diamond resists scratching better than any other gem, but a hard blow at the wrong angle can chip or cleave it. Thin girdles and pointed shapes like pear and marquise are the most vulnerable.
Are Lab-Grown Diamonds as Hard as Natural Diamonds?
Yes. Lab-grown diamonds have the same crystal structure of tightly bonded carbon atoms as natural diamonds, and GIA describes them as having essentially the same chemical, physical, and optical properties. Both rate 10 on the Mohs scale.
Where Do Moissanite and Cubic Zirconia Fall on the Mohs Scale?
Moissanite rates about 9.25, which places it above ruby and sapphire but below diamond. Cubic zirconia rates about 8 to 8.5, close to topaz.
Can You Test a Diamond by Scratching Glass?
No. Glass rates around 5.5 to 6.5 on the Mohs scale, so quartz, topaz, cubic zirconia, and moissanite all scratch it too. A glass scratch test can damage a stone without confirming it is a diamond. A jeweler or gem lab can identify the stone without harming it.
Is a 7 on the Mohs Scale Hard Enough for a Ring?
Yes, with realistic expectations. GIA rates amethyst, a 7, as suitable for rings as long as the wearer understands the limits of its hardness. A 7 matches the quartz in household dust, so polish and facet edges can soften over years of daily wear.
Where Do Gold, Platinum, and Silver Fall on the Mohs Scale?
In pure form, gold rates about 2.5, silver about 2.5 to 3, and platinum about 3.5. The alloys used in jewelry are harder, but they are still far softer than nearly every gemstone. That gap is why metal shows scratches long before a hard stone does.