The Mohs Scale Explained Simply: The Hardness of Gemstones
“Hardness 9 on the Mohs scale”: the phrase appears on every gemstone listing. But what does it really measure? And why is it THE figure to check before buying a stone meant to be worn? A simple explanation.
A scale invented in 1812… still in use
The German mineralogist Friedrich Mohs ranked ten reference minerals from the softest to the hardest, each one scratching the last: from talc (1) to diamond (10). Hardness therefore measures a stone's resistance to scratching — not to knocks, an important distinction.
Gems on the scale
- 10 — Diamond: nothing else scratches it;
- 9 — Sapphire and ruby (corundum): the champions of everyday wear;
- 8 — Spinel, topaz: excellent for all jewellery;
- 7.5-8 — Emerald, aquamarine, beryls;
- 7-7.5 — Tourmaline, garnet, amethyst, zircon;
- 6-6.5 — Tanzanite, moonstone, peridot: magnificent, but to be worn with care.
The magic threshold: quartz and dust
Why is 7 the critical threshold? Because everyday dust contains quartz (hardness 7). Any stone softer than 7 will pick up micro-scratches simply from being wiped or rubbed over the years — its polish grows dull. Above 7, the stone stands up to daily life.
Careful: hardness ≠ toughness
A very hard stone can still be sensitive to knocks: that is toughness, a different property. Emerald (7.5-8) is hard to scratch but dreads impacts because of its natural inclusions. Hence the importance of the setting: a bezel setting or protective claws change everything for delicate stones — our partner bench jeweller tailors every mount to the stone chosen.
In practice, for your purchase
A ring worn every day: aim for 8 or above (sapphire, ruby, spinel). A pendant or earrings: anything goes, even a tanzanite or a moonstone. Unsure about your project? Let's talk it over in Paris.
