Mohs Hardness Scale Converter
Use this Mohs hardness scale converter to translate between the classic 1–10 Mohs scale and a more quantitative relative (absolute) hardness scale. Explore how hard common minerals and gemstones are, and compare their scratch resistance for geology, gemology and materials science applications.
This tool is designed for quick, educational conversions and field work. For critical engineering design, always rely on standardized hardness tests (Vickers, Rockwell, Brinell) performed on the actual material.
Interactive Mohs hardness converter
You can enter decimal values (e.g. 6.5) to represent materials between two reference minerals.
Absolute hardness numbers are relative values used to compare resistance to scratching between Mohs levels.
Results
- Mohs hardness (approx.):
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- Absolute hardness (approx.):
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- Classification:
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- Closest reference mineral:
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Enter a value and click “Convert” to see the approximate hardness and closest Mohs reference mineral.
Important: This converter uses a reference table and simple interpolation. Values are indicative only and should not replace standardized hardness testing for critical design or certification.
Mohs hardness scale reference table
The table below lists the ten classic Mohs reference minerals, their hardness level and an approximate absolute hardness value used in this converter.
| Mohs | Reference mineral | Approx. absolute hardness | Typical use / notes |
|---|---|---|---|
| 1 | Talc | 1 | Very soft; marks easily with a fingernail. |
| 2 | Gypsum | 2 | Soft; can be scratched with a fingernail. |
| 3 | Calcite | 14 | Scratched by a copper coin; common in limestones. |
| 4 | Fluorite | 21 | Medium hardness; used in industry and optics. |
| 5 | Apatite | 48 | Benchmark for medium-hard minerals. |
| 6 | Orthoclase feldspar | 72 | Scratches glass; common rock-forming mineral. |
| 7 | Quartz | 100 | Very common; reference point for many abrasives. |
| 8 | Topaz | 200 | Hard gemstone; suitable for jewellery. |
| 9 | Corundum | 400 | Includes ruby and sapphire; used as an abrasive. |
| 10 | Diamond | 1500 | Extremely hard; cutting tools, high-wear applications. |
Absolute hardness numbers are dimensionless values that reflect the relative gaps between Mohs levels. They are not a substitute for standardized indentation hardness tests but help visualize how hardness increases along the Mohs scale.
How the Mohs hardness scale works
The Mohs scale is an ordinal scratch scale from 1 (very soft) to 10 (very hard). A mineral with a higher Mohs number can visibly scratch one with a lower number. For example, quartz (7) can scratch orthoclase (6), but not corundum (9) or diamond (10).
Because the scale is ordinal, the gap between 9 and 10 is much larger than the gap between 3 and 4. In other words, diamond is far harder than corundum, even though their Mohs values differ by only one step.
Key idea:
If mineral A scratches mineral B, A is harder than B on the Mohs scale. If neither scratches the other, they have similar hardness.
Mohs hardness vs. engineering hardness tests
In materials science and mechanical design, hardness is usually measured with indentation tests such as Vickers, Rockwell or Brinell. These methods use a controlled load and an indenter geometry to measure how deeply a material is penetrated.
- Mohs hardness: scratch test, quick and simple, qualitative and ordinal.
- Vickers, Rockwell, Brinell: indentation tests, quantitative, tied to specific standards.
Because the test methods are very different, there is no single exact formula that converts Mohs hardness into engineering hardness numbers. The best you can do is use approximate correlations for a given class of materials (for example, steels or specific minerals).
Classification of materials by Mohs hardness
A very rough rule of thumb often used in geology and mineralogy is:
- Soft: Mohs 1–2 (talc, gypsum)
- Medium: Mohs 3–5 (calcite, fluorite, apatite)
- Hard: Mohs 6–7 (orthoclase, quartz)
- Very hard: Mohs 8–10 (topaz, corundum, diamond)
Our converter uses these ranges to display a plain-language classification for any entered value.
How to use this Mohs hardness converter in practice
1. Identifying unknown minerals
In the field, geologists often use a scratch kit or simple objects (fingernail, copper coin, steel knife, glass plate) to estimate Mohs hardness by trial and error. Once you have an approximate Mohs value, you can use this converter to:
- see the closest reference mineral,
- compare its absolute hardness with other minerals,
- decide whether a sample is likely to scratch or be scratched by common materials.
2. Gemstone durability and jewellery design
Gemologists and jewellers use the Mohs scale to evaluate whether a gemstone is suitable for daily wear. As a very broad guideline:
- Everyday rings and bracelets usually require gemstones around Mohs 7 or higher.
- Softer stones (Mohs 5–6) may still be acceptable in protected settings such as earrings and pendants.
- Very soft stones (Mohs < 5) are generally best reserved for occasional wear or collectors.
3. Abrasives and surface finishing
When selecting abrasives, coatings or cutting tools, you need a material that is harder than the workpiece. A quick Mohs comparison can help you decide whether a given mineral abrasive is appropriate before checking more precise data in engineering hardness scales.
Limitations and good practice
- Mohs hardness is measured on ideal, pure crystals. Real-world rocks and industrial materials may behave very differently because they are mixtures or contain defects.
- The converter’s absolute hardness numbers are approximate and should be used for comparison only, not for design calculations.
- For critical applications (structural components, high-speed cutting tools, safety-relevant parts), always rely on certified hardness tests performed on the actual material grade.