Mohs' Scale of Hardness

Mohs' scale ranks the scratch resistance of various minerals by the ability of a harder material to scratch a softer material. It was created, in 1812, by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science. Mohs based the scale on ten minerals that are all readily available except the last one, diamond. 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.

The table below includes absolute hardness measured by a sclerometer. with, for example, corundum being twice as hard as topaz, but diamond, almost four times as hard as corundum. Mohs' is a purely ordinal scale.

Mohs Mineral   Absolute Hardness
hardest 10 diamond   1500
  9 corundum Ruby,Sapphire 400
  8 topaz Spinel 200
  7.5 beryl Emerald,Aquamarine  
  7 quartz Amethyst Citrine Iolite Tourmaline 100
  6 feldspar Opal Turquoise 72
  5 apatite   48
  4 fluospar platinum 21
  3 calcite   9
  2 gypsum gold,silver 2
softest 1 talc   1