The Mohs scale measures the relative hardness of minerals by scratch resistance.
German geologist and mineralogist Friedrich Mohs introduced the scale in 1812. It ranks ten reference minerals from 1, the softest, to 10, the hardest. A mineral with a higher number can scratch a mineral with a lower number, although the scale does not represent equal steps in hardness.
Talc is the reference mineral at 1, while diamond is the reference mineral at 10. Quartz, assigned 7, can scratch ordinary glass, which is why simple field tests often use a glass plate. A steel nail and a copper coin can provide rough comparisons for softer specimens.
Mohs hardness is different from toughness, cleavage, and resistance to breaking. A mineral may be hard yet brittle, or relatively soft yet difficult to break. The scale is especially useful for identification because it requires inexpensive tools, but precise scientific comparisons use quantitative tests such as Vickers or Knoop hardness measurements.