- Pronunciation
- MOHZ skayl
Mohs scale of mineral hardness — a comparative ordinal scale for the scratch resistance of solid materials, developed by the German mineralogist Friedrich Mohs in 1812. The scale runs from 1 (talc) to 10 (diamond). A higher-numbered mineral can scratch a lower-numbered one, but the intervals between numbers are not linear: the absolute hardness gap between 9 (corundum) and 10 (diamond) is much larger than the gap between 1 and 9 combined.
The reference minerals
| Mohs | Reference mineral |
|---|---|
| 1 | Talc |
| 2 | Gypsum |
| 3 | Calcite |
| 4 | Fluorite |
| 5 | Apatite |
| 6 | Orthoclase feldspar |
| 7 | Quartz |
| 8 | Topaz |
| 9 | Corundum (sapphire, ruby) |
| 10 | Diamond |
Where common gems fall
- 10 — Diamond.
- 9 — Sapphire, ruby (corundum).
- 8.5 — Alexandrite, chrysoberyl.
- 8 — Topaz, spinel.
- 7.5–8 — Aquamarine, emerald (beryl).
- 7–7.5 — Garnet, tourmaline.
- 6.5–7 — Tanzanite, peridot, andalusite, color-change diaspore (Zultanite).
- 6–6.5 — Moonstone, opal (depending on type).
- 5.5 — Lapis lazuli, turquoise (typical).
Practical implications for jewelry
The standard threshold for “everyday-wear hardness” in ring stones is conventionally Mohs 7 — quartz, the abrasive in airborne dust, sits at 7. A stone harder than quartz resists scratching from common environmental exposure; a stone softer than quartz does not. Zultanite at 6.5–7 sits at the borderline. With protected settings and care, it serves as a daily-wear stone; in exposed mountings or rough use, it accumulates abrasion faster than corundum or chrysoberyl.
The Mohs scale measures only scratch resistance, not toughness. A stone can be hard but brittle (diamond is the classic example — Mohs 10 but vulnerable to cleavage on impact). Cleavage, fracture toughness, and chemical stability are independent properties that complement Mohs hardness in a complete durability assessment.
Limitations of the scale
The Mohs scale is ordinal, not ratio-based. The Vickers and Knoll absolute hardness scales — measured by indentation under a controlled load — give linear hardness values and reveal the true non-linearity hidden in the Mohs ranking. Diamond’s Vickers hardness is roughly 4 to 5 times that of corundum; the Mohs scale compresses that to a difference of one unit.
See also: color change, diaspeírō.