Mohs Hardness
Also: Mohs scale, Scratch hardness, Mineral hardness
Mohs hardness is a dimensionless figure on a scale from 1 to 10 that describes a mineral's resistance to being scratched by a harder material.
Mohs hardness (also the Mohs scale or scratch hardness) was developed in 1812 by the German mineralogist Friedrich Mohs and remains to this day the most widely used qualitative measure of the scratch resistance of minerals and metals. The principle is remarkably simple: a material with a higher Mohs hardness scratches a material with a lower one - and not the other way around. This makes the scale an excellent quick reference tool in coin authenticity testing and in materials science.
The Mohs scale at a glance
The scale runs from 1 (talc, the softest mineral) to 10 (diamond, the hardest natural material). It is not linear - the intervals between the steps differ in size, which is why no arithmetic interpolation is permissible.
| Step | Reference mineral | Everyday comparison |
|---|---|---|
| 1 | Talc | Crumbles under a fingernail |
| 2 | Gypsum | Scratched by a fingernail |
| 3 | Calcite | Scratched by a copper coin |
| 4 | Fluorite | Scratched by a knife (easily) |
| 5 | Apatite | Scratched by a knife (with difficulty) |
| 6 | Feldspar | Scratched by a steel file |
| 7 | Quartz | Scratches glass |
| 8 | Topaz | Scratches quartz |
| 9 | Corundum (sapphire/ruby) | Scratches topaz |
| 10 | Diamond | Scratches everything |
Precious metals on the Mohs scale
Precious metals, with Mohs hardnesses between 2.5 and 4.75, are comparatively soft, which accounts for their characteristic malleability (ductility) but also explains their susceptibility to scratches:
| Metal | Mohs hardness (approx.) |
|---|---|
| Gold (999) | 2.5 |
| Silver (999) | 2.5-3.0 |
| Platinum | 3.5-4.5 |
| Palladium | 4.75 |
| Tungsten | 7.5 |
The low Mohs hardness of fine gold and fine silver is an important reason why these metals are alloyed with copper or other metals for coins and jewellery: an alloy noticeably increases the hardness without significantly impairing the precious-metal properties.
Significance for authenticity testing
Mohs hardness is a useful but not a sufficient authenticity criterion. Tungsten (Mohs hardness 7.5) is considerably harder than gold, yet it is used as a core in counterfeit bars because its density (19.25 g/cm3) is almost identical to that of gold (19.32 g/cm3). The tungsten counterfeit can therefore be reliably detected neither by scratch hardness alone nor by weighing alone - only a combination of density measurement, ultrasound and X-ray fluorescence provides certainty.
For simple everyday coin checks the rule is: a genuine gold coin can easily be scratched by a steel knife (Mohs hardness ~5.5) - something you should of course never actually try on it. The coin authenticity check and the coin weight checker offer safer, non-destructive alternatives.
Measuring principle
Hardness A > Hardness B → A scratches B, B does not scratch A
Hardness A = Hardness B → both scratch each other
Measurement is carried out by manually pressing the test material against the sample piece. It is then checked whether a genuine groove has formed or merely abrasion from the softer test body.
In brief
Mohs hardness ranks materials by their scratch resistance on a scale from 1 to 10 and helps to distinguish precious metals from counterfeits or alloying partners - for a reliable gold price valuation or authenticity test it should always be combined with further physical characteristics such as density and melting point.