Field report · 593
Mohs hardness scale, explained for buyers
The 10-point scale that tells you which stones can take a daily ring, and which need to be displayed under glass.
The Mohs hardness scale was developed in 1812 by Friedrich Mohs to rank minerals by their resistance to scratching. It's the most-used (and most-misunderstood) durability measure in the gem trade.
The scale
| Hardness | Reference mineral | What can scratch it |
|---|---|---|
| 1 | Talc | Fingernail (easily) |
| 2 | Gypsum (selenite) | Fingernail |
| 3 | Calcite | Copper coin |
| 4 | Fluorite | Knife blade |
| 5 | Apatite | Knife blade (with effort) |
| 6 | Orthoclase feldspar | Steel file |
| 7 | Quartz | Hardened steel |
| 8 | Topaz | Sapphire |
| 9 | Corundum (ruby/sapphire) | Diamond |
| 10 | Diamond | Another diamond |
What the scale really measures
Mohs measures scratch resistance only. It does NOT measure:
- Toughness — resistance to chipping or breaking. Topaz (Mohs 8) is harder than quartz (Mohs 7) but more prone to chipping due to its perfect cleavage.
- Stability — resistance to heat, acid, light, water
- Durability under wear — depends on hardness + toughness + cleavage + use case
Practical guidance for buyers
By use case:
- Daily-wear ring — Mohs 7+ recommended (sapphire 9, ruby 9, diamond 10, topaz 8, quartz family 7, tourmaline 7-7.5)
- Pendant / earring (low impact) — Mohs 5+ acceptable (lapis 5-6, malachite 3.5-4 with caveats, turquoise 5-6)
- Display only — any hardness works
- Avoid cleaning ultrasonically — any stone with cleavage (topaz, emerald, fluorite), any treated stone, anything below Mohs 7 unless specifically rated
Common misconceptions
- "Mohs is linear" — no, it's logarithmic-ish. Diamond is ~150x harder than corundum on absolute scratch tests.
- "Higher Mohs = more durable" — only partly. Jade (Mohs 6.5-7) is one of the toughest stones because of its interlocking microcrystalline structure. Diamond is the hardest mineral but can chip.
- "Quartz is too soft for rings" — wrong. Mohs 7 is the daily-wear threshold; most quartz family stones (amethyst, citrine, smoky) are fine in rings with reasonable care.