The Mohs Scale and Why Hardness Is Not Toughness
Mohs hardness measures scratch resistance only. A hard stone can still chip, which is what decides whether it belongs in a ring.
The Mohs scale ranks ten minerals by which one scratches which, from talc at 1 to diamond at 10, and it describes resistance to scratching and nothing else.
The scale
| Hardness | Mineral | Familiar stones at this level |
|---|---|---|
| 1 | Talc | Talc |
| 2 | Gypsum | Amber, and pearl below this |
| 3 | Calcite | Calcite |
| 4 | Fluorite | Fluorite |
| 5 | Apatite | Apatite, and most treated turquoise |
| 6 | Orthoclase feldspar | Moonstone, labradorite, tanzanite at 6.5 |
| 7 | Quartz | Amethyst, citrine, agate, and garnet at 6.5 to 7.5 |
| 8 | Topaz | Topaz, spinel, and beryl at 7.5 to 8 |
| 9 | Corundum | Ruby and sapphire |
| 10 | Diamond | Diamond |
The scale is a comparison rather than a measurement in units, so the numbers are ranks rather than quantities. The steps are not equal. Diamond is roughly four times harder than corundum in absolute terms, though the numbers are one apart. Between 6 and 7 there is a large practical gap, because quartz is the most common abrasive in household dust.
Hardness is not toughness
Toughness is resistance to breaking, chipping, and cleaving. It is a separate property, and the two do not track together.
Diamond is the hardest mineral known and has perfect cleavage in four directions. A sharp knock along one of those planes can split a diamond. Topaz is Mohs 8 with one perfect cleavage direction, which makes it one of the most dangerous hard stones to set in a ring. Emerald is Mohs 7.5 to 8, and the internal fractures that give it its color also make it prone to chipping at facet edges.
At the other end, jade is not at the top of the scale. Nephrite sits around Mohs 6 to 6.5 and jadeite around 6.5 to 7, yet both are exceptionally tough because their crystals interlock in a dense felted structure rather than cleaving along a single plane.
Dust is an abrasive
Fine quartz dust is everywhere and sits at Mohs 7. Loose stones rolling together in a box will scratch anything softer, and a jewelry pouch that once held a quartz stone can scratch a softer one later. This is why storage means separating stones rather than piling them together.
Which stones are risky in rings
A ring worn daily meets doorknobs, car doors, desks, and gym equipment. On that schedule:
- Stones at Mohs 7 or above are the practical range. Corundum, diamond, spinel, topaz, quartz, and garnet all qualify, with the caveats above about cleavage.
- Stones between 6 and 7 need a protective setting and some care. Tanzanite, peridot, moonstone, and labradorite live here.
- Stones below 6 will abrade, chip, or lose polish in a ring worn often. Opal, fluorite, apatite, turquoise, pearl, amber, and malachite are all finer in pendants, earrings, and brooches.
Treatments can lower the effective toughness of a stone regardless of its hardness. A ruby with a glass-filled fracture is a corundum, at Mohs 9, and the filled area is not.
For a ring you intend to wear every day, stay at Mohs 7 or above and check whether the species has a perfect cleavage before you commit.
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