The Mohs scale of mineral hardness is one of the most practical and enduring tools in the world of geology. Created in 1812 by the German mineralogist Friedrich Mohs, this qualitative scale ranks minerals from 1 to 10 based on their ability to scratch one another.

How the Scale Works

The Mohs scale is a 'scratch test.' If Mineral A can scratch Mineral B, then Mineral A is harder than Mineral B. It is important to note that the scale is relative, not absolute. For example, Diamond (10) is actually four times harder than Corundum (9), even though they are adjacent on the scale.

The 10 Reference Minerals

Friedrich Mohs selected ten common minerals as benchmarks for his scale:

1. Talc: The softest mineral. Can be scratched by a fingernail. 2. Gypsum: Can be scratched by a fingernail. 3. Calcite: Can be scratched by a copper coin. 4. Fluorite: Can be scratched by a steel knife. 5. Apatite: Can be scratched by a steel knife, but with difficulty. 6. Orthoclase: Can scratch glass but can be scratched by a steel file. 7. Quartz: Scratches glass easily. 8. Topaz: Scratches quartz easily. 9. Corundum: (Rubies and Sapphires) Scratches almost everything. 10. Diamond: The hardest natural substance on Earth.

Common Objects for Testing

You don't always need a set of reference minerals to test hardness. Geologists often use everyday objects:

  • Fingernail: ~2.5
  • Copper Penny: ~3.5
  • Steel Knife/Glass: ~5.5
  • Steel File: ~6.5

Why Hardness Matters

Understanding mineral hardness is critical for several reasons:

1. Mineral Identification

Hardness is a diagnostic property. If you find a clear crystal that you can scratch with your fingernail, it's definitely not quartz—it's likely gypsum.

2. Jewelry and Durability

Gemstones used in rings are typically 7 or higher on the Mohs scale. This ensures they can withstand daily wear without becoming dull or scratched by common household dust (which often contains tiny particles of quartz).

3. Industrial Applications

Harder minerals are used as abrasives. Garnet, corundum, and industrial diamonds are used in sandpaper, cutting tools, and polishing compounds.

Conclusion

The Mohs scale remains a vital part of every geologist's field kit. Its simplicity and reliability make it the perfect starting point for anyone looking to identify and understand the incredible variety of minerals that make up our planet.