The Mohs scale of mineral hardness is a qualitative ordinal
scale which characterizes the scratch resistance of various
minerals through the ability of a harder material to scratch a
softer material.

The hardness of a material is measured against the scale
by finding the hardest material that the given material can
scratch, and/or the softest material that can scratch the
given material. For example, if some material is scratched
by apatite but not by fluorite, its hardness on the Mohs
scale would fall between 4 and 5.