The Mohs Hardness Scale is one of the most fundamental tools in mineral identification. Developed by German geologist Friedrich Mohs in 1812, this scale measures a mineral's resistance to scratching relative to other common minerals.
Understanding the Mohs Scale
The Mohs scale is a relative scale from 1 (softest) to 10 (hardest), with each number representing a specific mineral. A mineral can scratch any other mineral with a lower number on the scale.
| Hardness | Mineral | Common Objects with Similar Hardness |
|---|---|---|
| 1 | Talc | Fingernail (2.5) |
| 2 | Gypsum | |
| 3 | Calcite | Copper coin (3.5) |
| 4 | Fluorite | Iron nail (4.5) |
| 5 | Apatite | Glass (5.5-6.0) |
| 6 | Orthoclase Feldspar | Steel file (6.5) |
| 7 | Quartz | |
| 8 | Topaz | |
| 9 | Corundum | Sapphire and ruby (varieties of corundum) |
| 10 | Diamond |
How to Perform a Hardness Test
1. Gather Your Materials
- Unknown mineral sample
- Hardness testing kit or common objects of known hardness
- Hand lens for observing scratches
2. Conduct the Test
To determine a mineral's hardness:
- Select a sharp corner of the test object (or known hardness mineral)
- Gently but firmly scratch the surface of the unknown mineral
- Examine the result: if the unknown mineral is scratched, it has a lower hardness than the test object; if the test object leaves no mark, the unknown mineral has a higher hardness
- Repeat with objects of increasing or decreasing hardness to narrow down the range
3. Tips for Accurate Testing
- Always test on a fresh surface, not on weathered material
- Be careful not to confuse a mineral's cleavage planes with scratches
- Clean the sample before testing to remove any dirt or debris
- Use gentle pressure to avoid breaking fragile samples
Why Hardness Testing Matters
Hardness is one of the most reliable physical properties for mineral identification. It helps geologists and collectors quickly narrow down the possibilities when examining unknown specimens. In addition, a mineral's hardness provides clues about its atomic structure and can help determine its potential uses in industry.
Limitations of the Mohs Scale
While extremely useful, the Mohs scale has some limitations:
- It is a relative scale, not a linear or quantitative measure of hardness
- Some minerals may have different hardness in different directions (pleochroism)
- Fine-grained or porous minerals may give inaccurate results