The Mohs Hardness Scale
The Mohs Hardness Scale is a standard way to measure the relative hardness of minerals (and by extension, many rocks). It's based on ten minerals, each of which can scratch the ones below it on the scale:
1. Talc (very soft, can be scratched by a fingernail)
2. Gypsum (soft, 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 be scratched by a steel file)
7. Quartz (can scratch glass)
8. Topaz
9. Corundum (can scratch all minerals except diamond)
10. Diamond (the hardest known natural mineral)
Using the Mohs Hardness Scale to Identify Rocks
1. Scratch Test: Carefully try to scratch the rock with known objects. You can use items like a fingernail, a copper coin, a steel knife, or a piece of glass.
2. Compare: If the object scratches the rock, the rock's hardness is *less than* the object's hardness. If the rock scratches the object, its hardness is *greater than* the object's.
3. Narrowing Down: By comparing the rock's hardness to known objects, you can narrow down the potential minerals in the rock.
Important Considerations:
* Mineral Composition: Rocks are made of multiple minerals. The hardness you determine will be influenced by the hardest mineral present.
* Surface Variations: The hardness of a rock can vary depending on the surface you're testing (e.g., a fractured surface might be less hard).
* Not the Only Factor: Hardness is just one property used for rock identification. Other factors, such as color, luster, cleavage, and crystal shape, are crucial too.
Example:
Let's say you find a rock that can be scratched by a steel knife but not by a copper coin. This means the rock's hardness is somewhere between 3 (Calcite) and 5 (Apatite). Further testing with a steel file or other tools could help you pinpoint a more accurate hardness.
Beyond Hardness:
While helpful, hardness alone isn't sufficient for rock identification. You'll need to consider other characteristics and consult a field guide or online resource for accurate identification.
