Hardness   Mineral          Absolute Hardness
  1 Talc (Mg3Si4O10(OH)2)             1
  2 Gypsum (CaSO4·2H2O)               2
  3 Calcite (CaCO3)                   9
  4 Fluorite (CaF2)                  21
  5 Apatite (Ca5(PO4)3(OH-,Cl-,F-)   48
  6 Orthoclase Feldspar (KAlSi3O8)   72
  7 Quartz (SiO2)                   100
  8 Topaz (Al2SiO4(OH-,F-)2)        200
  9 Corundum (Al2O3)                400
 10 Diamond (C)                    1500

On the Mohs scale, a pencil lead has a hardness of 1;
a fingernail has hardness 2.5; a copper penny, about 3.5;
a knife blade, 5.5; window glass, 5.5; steel file, 6.5.
[1] Using these ordinary materials of known hardness
can be a simple way to approximate the position of a
mineral on the scale.