Gemstone Hardness: The Mohs Scale, Toughness and Stability
The Mohs hardness scale and its ten reference minerals, hardness of common gems, how toughness and stability differ, why dust scratches gems, and wear advice.
Gemstone hardness is a stone’s resistance to scratching, and jewelers rate it on the Mohs scale from 1 (talc) to 10 (diamond). Hardness is only one part of durability: toughness describes resistance to chipping and breaking, and stability describes resistance to heat, light, chemicals and humidity. A gem that scores well on all three suits everyday wear; one that is soft, brittle or sensitive needs a protective setting and gentler use.
The Mohs scale
Friedrich Mohs introduced his scale in 1822. As GIA’s durability guide explains, he chose ten minerals and numbered them by how easily one could scratch another. Each mineral scratches those below it and is scratched by those above.
| Mohs number | Reference mineral | Gem examples near this hardness |
|---|---|---|
| 1 | Talc | None used in jewelry |
| 2 | Gypsum | Pearl (2.5 to 3) |
| 3 | Calcite | Pearl, upper end |
| 4 | Fluorite | Malachite (about 4) |
| 5 | Apatite | Lapis lazuli (about 5); turquoise and opal start here |
| 6 | Orthoclase feldspar | Moonstone (6 to 6.5), tanzanite (6 to 7) |
| 7 | Quartz | Amethyst and citrine (7), garnet (6.5 to 7.5) |
| 8 | Topaz | Topaz and spinel (8), aquamarine and emerald (7.5 to 8) |
| 9 | Corundum | Ruby and sapphire |
| 10 | Diamond | Diamond |
The reference minerals follow the USGS list. Gem values are from GIA’s birthstone and care guides and, for malachite and lapis, from a Ganoksin table of common gems.
The scale is not evenly spaced
Mohs numbers are a ranking, not a measurement. GIA stresses that the steps are uneven: diamond at 10 is many times harder than corundum at 9, even though they sit one number apart. A stone rated 7.5 is harder than one rated 7, but the numbers say nothing about how much harder.
Common gems by hardness
Grouping gems by hardness gives a quick guide to how they will hold their polish.
| Group | Gems (Mohs, per GIA unless noted) |
|---|---|
| 9 and above | Diamond (10); ruby and sapphire (9) |
| 8 to 8.5 | Alexandrite (8.5); chrysoberyl (8.5, Ganoksin); topaz and spinel (8) |
| 7 to 8 | Aquamarine and emerald (7.5 to 8); tourmaline (7 to 7.5); amethyst and citrine (7) |
| 6 to 7.5 | Garnet (6.5 to 7.5); zircon (6 to 7.5); peridot, bloodstone and sardonyx (6.5 to 7); tanzanite (6 to 7); moonstone (6 to 6.5) |
| Below 6.5 | Opal (5 to 6.5); turquoise (5 to 6); pearl (2.5 to 3) |
Ganoksin’s table puts organic gems such as pearl and amber generally between 2.5 and 4.
Hardness, toughness and stability
GIA defines durability as a gem’s ability to withstand wear, heat, light, household chemicals and changes in humidity. It breaks that into three properties.
Hardness
Hardness is resistance to scratches and abrasion. A soft gem’s facet edges gradually round off and its surface goes dull, even if the stone never chips.
Toughness
Toughness is a gem’s ability to resist breaking, chipping or cracking when struck. It depends on how the crystal is bonded and is quite separate from hardness. Examples from GIA:
- Diamond is the hardest gem but not the toughest; a hard blow can cleave or fracture it.
- Jade, both nephrite and jadeite, is exceptionally tough, far tougher in GIA’s comparison than corundum.
- Ruby, sapphire and alexandrite combine high hardness with excellent toughness and no cleavage.
- Topaz is hard at 8 but, per GIA’s November guide, has poor toughness and can chip or crack.
- Moonstone has poor toughness and may crack when exposed to high heat, according to GIA’s June guide.
Cleavage, in GIA’s words, is a tendency to break when struck, along planes in the crystal. Stones that can cleave, diamond included, need care during setting and repair.
Stability
Stability is resistance to heat, light, chemicals and humidity. GIA’s examples include:
- Opal, which can lose moisture and crack or craze when humidity changes.
- Pearl, which chlorine, perfume and makeup can damage.
- Amethyst, which can fade in prolonged intense light.
- Emerald, which should be kept from heat, changes in air pressure and harsh chemicals, partly because of the fillers many stones contain.
- Topaz, which can develop internal fractures from high heat or sudden temperature changes.
GIA’s care guides warn about high heat again and again, so stones should be identified and, where needed, removed before any torch repair.
Why dust matters
Ordinary dust is abrasive. GIA notes that dust often contains quartz, and quartz ranks 7 on the Mohs scale, so gems with a hardness of 7 or lower can be scratched by it. Ganoksin’s table makes the same point, adding that nearly all gemstones can be scratched by dust.
That has practical consequences:
- Don’t wipe a dusty stone dry. Rinse grit off first, then clean. Rubbing drags quartz grains across the surface.
- Expect wear on soft stones. Opal, turquoise, pearl and similar gems will lose polish over years of daily wear even with careful owners.
- Store soft stones apart. A pouch or separate compartment keeps them away from both grit and harder gems.
Cleaning methods by stone are covered in How to Clean Jewelry Safely.
Everyday-wear recommendations
GIA’s general advice is to reserve softer gems for special occasions rather than daily wear. In practice:
| Use | Good choices | Use with care |
|---|---|---|
| Daily-wear rings | Diamond, ruby, sapphire, alexandrite, spinel, aquamarine | Emerald, topaz (protective setting) |
| Occasional rings | Garnet, amethyst, citrine, tourmaline | Tanzanite, zircon, peridot |
| Earrings and pendants | Nearly any gem | Pearl and opal still need care with cosmetics and heat |
A few further points help match stone to wearer:
- Setting style matters. A bezel protects the girdle and edges of a softer or brittle stone better than open prongs. Bench technique is covered in Bezel-Setting a Round Brilliant.
- Treatments change durability. Fracture-filled, dyed or coated stones often need gentler handling than untreated stones of the same species, and sellers should disclose such treatments.
- Store pieces apart. Harder stones, diamond above all, will scratch softer ones in a shared box.
For month-by-month stones and their individual care notes, see Birthstones by Month. Tanzanite, at 6 to 7, is a good example of a popular gem that rewards protective settings; its sources and buying points are in Tanzanite: Where It Comes From and How to Buy It.
Common questions
Is diamond unbreakable because it is a 10? No. Hardness measures scratch resistance only. GIA notes that diamond can be cleaved or fractured by a hard blow.
Is a higher Mohs number always better for a ring? Not on its own. Topaz is an 8 but has poor toughness, while jade is softer yet exceptionally tough. Hardness, toughness and stability all count.
Why does my opal look cloudy or cracked? Opal is sensitive to humidity and heat. GIA notes it can lose moisture and craze, so it should be kept from sudden temperature changes and very dry conditions.
