What is Mohs hardness scale, and how is it used?

Short Answer:

Mohs hardness scale is a simple way to measure the scratch resistance of a material. It ranks materials from 1 to 10 based on their ability to resist being scratched by another material. A material with a higher number can scratch a material with a lower number, but not the other way around. For example, diamond is the hardest material with a value of 10, while talc is the softest with a value of 1.

This scale is mostly used to compare the hardness of minerals, but it is also helpful in industries like jewelry, mining, and geology. It helps in identifying unknown minerals, selecting cutting tools, and checking surface wear in various applications.

Detailed Explanation:

Mohs Hardness Scale and Its Uses

In mechanical engineering, geology, and material science, hardness is a key property that tells how well a material resists being scratched or worn. One of the oldest and simplest ways to measure hardness is the Mohs hardness scale. It is not used for precise laboratory testing but is very helpful for quick field identification and material comparison.

What is Mohs Hardness Scale?

The Mohs scale was introduced by Friedrich Mohs, a German geologist, in 1812. It ranks minerals on a scale of 1 to 10, based on their ability to scratch or be scratched by another substance.

  • A harder material can scratch a softer one.
  • A softer material cannot scratch a harder one.

Here are the original ten reference minerals in order:

  1. Talc – very soft (used in baby powder)
  2. Gypsum – can be scratched with a fingernail
  3. Calcite – can be scratched with a copper coin
  4. Fluorite
  5. Apatite
  6. Orthoclase (Feldspar) – can scratch glass
  7. Quartz
  8. Topaz
  9. Corundum – includes sapphire and ruby
  10. Diamond – the hardest known natural material

This scale is relative, not linear. This means that the gap in hardness between 9 and 10 is much larger than the gap between 2 and 3.

How is the Mohs Scale Used

  1. Mineral Identification in Geology
    Geologists use the scale to identify unknown rocks or minerals by scratching them with known materials. For example, if a mineral can be scratched by fluorite (4) but not by calcite (3), then its hardness is between 3 and 4.
  2. Tool and Blade Selection
    Cutting tools are made from materials harder than the workpiece. If you want to cut quartz (hardness 7), the tool must have a hardness greater than 7, like diamond (10) or carbide tools.
  3. Surface Wear Testing
    It helps to compare how quickly materials will wear out when rubbed or scratched. Softer materials are more likely to wear faster.
  4. Jewelry Industry
    The hardness of gemstones is measured using this scale to check how easily they can be scratched. Diamonds rank at the top, so they are used not just in jewelry, but also in cutting and grinding tools.
  5. Simple Hardness Check
    Engineers, craftsmen, and hobbyists use the Mohs scale for quick on-site checks without needing any complex instruments.

Benefits of Using Mohs Hardness Scale

  • Simple and easy: Does not require expensive equipment.
  • Quick results: Good for fieldwork or basic testing.
  • Useful for comparison: Helps choose materials for cutting, polishing, or wear resistance.

Limitations of the Mohs Scale

  • Not accurate for metals or modern industrial materials.
  • It is not precise or scientific because the scale is not equally spaced.
  • It only shows scratch hardness, not resistance to indentation or deformation.
  • It cannot be used to calculate actual stress or strain values.

For engineering materials like steel or aluminum, more accurate methods like Rockwell or Vickers hardness tests are used.

Example of Practical Use

Let’s say you find a shiny stone and want to know if it is quartz or calcite. You try to scratch it with a knife blade (hardness around 5.5). If the stone resists scratching, it may be quartz (7). If it gets scratched, it could be calcite (3). This way, the Mohs scale helps identify the material without any lab setup.

Conclusion

The Mohs hardness scale is a simple and effective method to compare how hard different materials are by seeing which one scratches the other. It is widely used in geology, gemology, and basic field testing to identify minerals and choose suitable materials for cutting or wear applications. While it does not provide precise results, it remains a useful and easy-to-use tool for quick hardness comparison in real-life situations.