Mohs Hardness
Also: Mohs scale, Scratch hardness, Mineral hardness
Mohs hardness is a dimensionless index on a scale from 1 to 10 that describes the resistance of a mineral 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 commonly used qualitative measure of the scratch hardness 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 round. This makes the scale excellently suited as a quick orientation tool in coin authenticity checking and in raw-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 distance between the steps varies in size, which is why arithmetic interpolation is not permissible.
| Level | Reference mineral | Everyday comparison |
|---|---|---|
| 1 | Talc | Crumbles with a fingernail |
| 2 | Gypsum | Scratchable with a fingernail |
| 3 | Calcite | Scratchable with a copper coin |
| 4 | Fluorite | Scratchable with a knife (easily) |
| 5 | Apatite | Scratchable with a knife (with difficulty) |
| 6 | Feldspar | Scratchable with 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 scratching:
| 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 increases the hardness noticeably without significantly impairing the precious-metal properties.
Significance for authenticity checking
Mohs hardness is a useful but not a sufficient authenticity feature. Tungsten (Mohs hardness 7.5) is admittedly considerably harder than gold, but 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 following applies: a genuine gold coin can easily be scratched with a steel knife (Mohs hardness ~5.5) – which of course one should not try on it. The coin authenticity check and the coin weight checker offer safer, non-destructive alternatives.
Measurement 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 specimen. It is then checked whether a genuine groove has formed or merely abrasion of the softer test body.
In brief
Mohs hardness ranks materials by their scratch hardness 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 check it should always be combined with further physical features such as density and melting point.