The Mohs scale of mineral hardness is one of the most practical, enduring, and essential tools in the world of geology. Whether you are a professional geologist in the field or a hobbyist rockhound in your backyard, understanding this scale is key to identifying minerals.
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. But what does 'hardness' actually mean in this context, and how can you use it?
What is Mineral Hardness?
In mineralogy, hardness is defined specifically as a mineral's resistance to scratching. It is not the same as toughness (resistance to breaking/fracturing). For example, a diamond is the hardest mineral (10), meaning it cannot be scratched by anything else. However, diamond is brittle; if you hit it with a hammer, it will shatter. Conversely, Jade is softer (6-7) but extremely tough; you can hit it with a hammer and it will likely ring like a bell without breaking.
How the Scale Works
The Mohs scale is a relative 'scratch test.' The rule is simple: Mineral A is harder than Mineral B if A can scratch B.
It is important to note that the scale is ordinal, not linear. The steps between numbers are not equal. For example, Corundum (9) is twice as hard as Topaz (8), but Diamond (10) is four times harder than Corundum. The gap between 9 and 10 is massive compared to the rest of the scale.
The 10 Reference Minerals
Friedrich Mohs selected ten common, distinct minerals to serve as the benchmarks for his scale. Memorizing this list is a rite of passage for every geology student:
1. Talc (Hardness 1): The softest mineral. It feels greasy and can be easily scratched by a fingernail. It is used in baby powder. 2. Gypsum (Hardness 2): Also soft enough to be scratched by a fingernail. It is the main ingredient in drywall. 3. Calcite (Hardness 3): The main component of limestone. It can be scratched by a copper coin. 4. Fluorite (Hardness 4): Known for its cubic crystals. It can be scratched by a steel knife. 5. Apatite (Hardness 5): The mineral that makes up your teeth (specifically hydroxyapatite). It can be scratched by a knife, but with some difficulty. 6. Orthoclase Feldspar (Hardness 6): A very common rock-forming mineral. It can scratch glass and cannot be scratched by a standard steel knife. 7. Quartz (Hardness 7): The most common mineral on Earth's surface. It scratches glass easily and is the dividing line for gemstones (dust contains quartz, so gems softer than 7 scratch over time). 8. Topaz (Hardness 8): A hard gemstone that scratches quartz. 9. Corundum (Hardness 9): This includes Rubies and Sapphires. It is used in industrial sandpaper and abrasive tools. 10. Diamond (Hardness 10): The hardest natural substance on Earth. It can only be scratched by another diamond.
How to Perform the Hardness Test
You don't always need a set of reference minerals to test hardness. You can use common objects with known hardness values to narrow down the possibilities.
Common Testing Tools:
- Fingernail: ~2.5
- Copper Penny: ~3.5
- Iron Nail: ~4.5
- Window Glass: ~5.5
- Steel File: ~6.5
- Masonry Drill Bit: ~8.5
The Procedure: 1. Find a fresh surface: Weathered surfaces can be softer. Find a clean, unweathered face of the mineral. 2. Scratch: Attempt to scratch the mineral with your testing object (e.g., your fingernail). Press firmly. 3. Inspect: Check if a scratch was made. Rub the markāsometimes it's just powder from the tool rubbing off on the stone. If there is a distinct groove, the tool is harder. 4. Reverse: If the tool didn't scratch the mineral, try scratching the tool *with* the mineral. If the mineral scratches the glass plate, you know it is harder than 5.5. 5. Bracket: Continue until you find a value. If a mineral scratches glass (5.5) but is scratched by a steel file (6.5), its hardness is around 6.
Why Hardness Matters
Understanding mineral hardness is critical for several real-world applications:
1. Mineral Identification
Hardness is a key diagnostic property. If you find a clear, hexagonal crystal, it could be Quartz or Apatite. A quick scratch test on glass will tell you the answer (Quartz scratches glass; Apatite does not).
2. Jewelry and Durability
This is crucial for engagement rings. A stone worn every day hits door frames, keys, and countertops. Dust in the air is mostly silica (quartz, hardness 7). If you choose a ring stone softer than 7 (like Opal or Tanzanite), the dust will slowly abrade it, making it dull over time. This is why Diamond, Sapphire, and Ruby are preferred for daily-wear rings.
3. Industrial Applications
Harder minerals are used as abrasives. Garnet is used in waterjet cutting; diamond dust is used in saw blades for cutting concrete and stone.
Conclusion
The Mohs scale remains a vital part of every geologist's field kit. Its simplicity, requiring only a few pocket tools, makes it powerful. By understanding how to test for hardness, you unlock a major clue in the detective work of mineral identification.