Mohs Hardness
Also: Mohs Scale, Scratch Hardness, Mineral Hardness
The Mohs hardness is a dimensionless value on a scale of 1 to 10 that describes the resistance of a mineral to scratching by a harder material.
Mohs hardness (also known as the Mohs scale or scratch hardness) was developed in 1812 by German mineralogist Friedrich Mohs and remains the most widely used qualitative measure of the scratch hardness of minerals and metals to this day. The principle is straightforward: a material with a higher Mohs hardness scratches one with a lower value – and not vice versa. This makes the scale an excellent quick-reference tool in coin authenticity testing and mineralogy.
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 steps are unequal, so arithmetic interpolation is not valid.
| Step | Reference Mineral | Everyday Comparison |
|---|---|---|
| 1 | Talc | Can be crumbled with a fingernail |
| 2 | Gypsum | Can be scratched with a fingernail |
| 3 | Calcite | Can be scratched with a copper coin |
| 4 | Fluorite | Can be scratched with a knife (easily) |
| 5 | Apatite | Can be scratched with a knife (with difficulty) |
| 6 | Feldspar | Can be scratched 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 have Mohs hardness values between 2.5 and 4.75 and are therefore relatively soft, which explains their characteristic malleability (ductility) but also 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 hardness without substantially impairing the noble metal properties.
Significance for Authenticity Testing
Mohs hardness is a useful but not sufficient authenticity indicator. Tungsten (Mohs hardness 7.5) is significantly harder than gold, yet it is used as a core in counterfeit bars because its density (19.25 g/cm³) is nearly identical to that of gold (19.32 g/cm³). Tungsten forgeries therefore cannot be reliably detected by scratch hardness or weighing alone – only a combination of density testing, ultrasound, and X-ray fluorescence provides certainty.
For simple everyday coin testing: a genuine gold coin can be easily scratched with a steel knife (Mohs hardness ~5.5) – which is of course something you would not want to do to 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
The measurement is carried out by pressing the test material against the sample piece manually. It is then checked whether a true groove formed or merely abrasion from the softer test piece.
In Brief
Mohs hardness ranks materials by their scratch hardness on a scale of 1 to 10 and helps distinguish precious metals from counterfeits or alloying partners – for reliable gold price assessment or authenticity testing, it should always be combined with further physical characteristics such as density and melting point.