Mohs Hardness Scale: What It Means for Gemstones
The Mohs scale is a relative measure of mineral scratch resistance, numbered 1 to 10. A mineral at a higher number scratches any mineral below it: quartz (7) scratches feldspar (6), and diamond (10) scratches everything. The scale was devised by German mineralogist Friedrich Mohs in 1812 and remains the standard tool for identifying minerals and assessing gem durability.
How the Scale Works
Friedrich Mohs chose ten reference minerals — talc, gypsum, calcite, fluorite, apatite, orthoclase feldspar, quartz, topaz, corundum, and diamond — and ranked them by which could scratch which. The numbers indicate order, not proportion: the gap in hardness between corundum (9) and diamond (10) is far larger in absolute terms than the gap between any adjacent pair lower on the scale.
In practice, you do not need the ten reference minerals. Common objects serve as rough benchmarks:
| Object | Approximate Mohs |
|---|---|
| Fingernail | 2.5 |
| Copper coin | 3 |
| Steel nail or knife blade | 5.5 |
| Glass plate | 5.5 |
| Steel file | 6.5 |
| Hardened steel file | 7 |
A gemstone that a steel knife blade cannot scratch has a hardness of at least 6. A stone scratched by a fingernail is at or below 2.5.
Hardness of Stones on This Site
The table below lists the Mohs hardness of gemstones featured in the gemstones directory, with links to each stone's hub page. Where a mineral shows a range, this reflects variation across crystal orientations, compositions, or varieties.
| Stone | Mohs hardness | Hub |
|---|---|---|
| Diamond | 10 | /gemstones/diamond |
| Ruby | 9 | /gemstones/ruby |
| Sapphire | 9 | /gemstones/sapphire |
| Emerald | 7.5–8 | /gemstones/emerald |
| Aquamarine | 7.5–8 | /gemstones/aquamarine |
| Black tourmaline | 7–7.5 | /gemstones/black-tourmaline |
| Amethyst | 7 | /gemstones/amethyst |
| Citrine | 7 | /gemstones/citrine |
| Clear quartz | 7 | /gemstones/clear-quartz |
| Rose quartz | 7 | /gemstones/rose-quartz |
| Tiger's eye | 7 | /gemstones/tiger-eye |
| Garnet | 6.5–7.5 | /gemstones/garnet |
| Peridot | 6.5–7 | /gemstones/peridot |
| Jade | 6–7 | /gemstones/jade |
| Kyanite | 4.5–7 | /gemstones/kyanite |
| Labradorite | 6–6.5 | /gemstones/labradorite |
| Moonstone | 6–6.5 | /gemstones/moonstone |
| Opal | 5.5–6.5 | /gemstones/opal |
| Hematite | 5.5–6.5 | /gemstones/hematite |
| Obsidian | 5–5.5 | /gemstones/obsidian |
| Lapis lazuli | 5–6 | /gemstones/lapis-lazuli |
| Turquoise | 5–6 | /gemstones/turquoise |
| Fluorite | 4 | /gemstones/fluorite |
| Malachite | 3.5–4 | /gemstones/malachite |
| Howlite | 3.5 | /gemstones/howlite |
| Calcite | 3 | /gemstones/calcite |
| Lepidolite | 2.5–4 | /gemstones/lepidolite |
| Selenite | 2 | /gemstones/selenite |
Kyanite is the most unusual entry: its hardness is strongly direction-dependent (anisotropic), measuring approximately 4.5 when scratched along the length of the crystal and 6–7 when scratched across it. This is an intrinsic property of its crystal structure, not a flaw.
Why Hardness Matters for Jewelry and Care
Hardness determines how resistant a gem is to surface scratching in everyday wear:
- Hardness 7 and above is the general threshold for gemstones used in rings, which sustain the most abrasion. Quartz dust (hardness 7) is present in ordinary household dust; any stone softer than quartz will gradually acquire surface scratches over time from ambient dust alone.
- Hardness 5–6 stones — moonstone, labradorite, opal, lapis lazuli, turquoise — are better suited to pendants, earrings, and brooches where direct abrasion is less frequent.
- Hardness below 5 — calcite, howlite, lepidolite, selenite — requires careful storage away from harder stones, avoidance of abrasive cleaners, and occasional polishing to maintain surface appearance.
Selenite (Mohs 2) is also water-soluble: prolonged contact with water dissolves the surface, which is a separate concern from hardness alone.
Hardness Is Not the Same as Toughness
Mohs hardness measures resistance to scratching, not resistance to breaking. A hard stone can still be brittle: diamond, the hardest mineral, cleaves along specific crystal planes and can chip from a sharp blow. Jade (nephrite), rated 6–7, is actually tougher than diamond in the engineering sense because its interlocking fibrous crystal structure resists fracture, even though it would be scratched by quartz.
Toughness is why a jeweler may set a very hard but brittle stone — such as tanzanite (Mohs 6–7, but with pronounced cleavage) — in a protected bezel setting, while a tougher but softer stone like jade can withstand more impact despite its lower Mohs rating.
For an explanation of the broader mineralogical definitions that underpin hardness testing, see gemstone vs crystal vs mineral.