3. Hardness.—By the hardness of a mineral we mean the resistance which it offers to abrasion. But hardness is a purely relative term, calcite, for example, being hard compared with talc, but very soft compared with quartz. Hence mineralogists have found it necessary to select certain minerals to be used as a standard of comparison for all others, and known as the scale of hardness. These are arranged at nearly equal intervals all the way from the softest mineral to the hardest, as follows:—
Scale of Hardness.
(1) Talc.
(2) Gypsum.
(3) Calcite.
(4) Fluorite.
(5) Apatite.
(6) Orthoclase.
(7) Quartz.
(8) Topaz or Beryl.