Hardness
Definition
A measure of a mineral's resistance to scratching, typically measured using the Mohs scale (1-10). Diamond is the hardest natural mineral at 10.
Example
Quartz has a hardness of 7, meaning it can scratch glass but can be scratched by topaz.
The Mohs Scale of Hardness
Hardness is a measure of a mineral's resistance to abrasion or scratching. In 1812, German mineralogist Friedrich Mohs established the Mohs Scale, a relative scale from 1 (softest) to 10 (hardest). It is important to note that this scale is ordinal, not linear. Diamond (10) is actually four times harder than Corundum (9), while Corundum is only twice as hard as Topaz (8).
How to Test Hardness
Geologists often use a "scratch test" kit in the field:
- Fingernail: Hardness ~2.5
- Copper Penny: Hardness ~3.5
- Steel Knife/Nail: Hardness ~5.5
- Glass Plate: Hardness ~5.5-6.0
- Streak Plate: Hardness ~6.5
If a mineral can scratch glass, it is harder than 5.5 (likely Quartz, Feldspar, or a gemstone). If you can scratch it with your fingernail, it is very soft (like Talc or Gypsum). Testing hardness requires finding a fresh surface and applying firm pressure.