There are three general classes of crystals--calcareous, silicious and
gypsum--but by far the most important are the silicious crystals because
of their great hardness. These include quartz or rock crystal--which is
quite common--and the so-called _precious stones_, among which are the
diamond, rubies, sapphires, etc., a description of which will be found
in the Dictionary of Minerals.
To find the relative hardness of substances, a scale has been arranged,
beginning with the softest mineral (talc) and ending with the hardest
(diamond). The minerals of the scale, therefore, are so arranged that
each will scratch any other mineral of lower number in the scale, or be
scratched by any of higher number.
=SCALE OF HARDNESS=
MINERAL CHEMICAL NAME
1. Talc. }Can be scratched{ 1. Magnesium silicate.
2. Gypsum (or rocksalt). } by the { 2. Calcium sulphate or
} finger-nail { Sodium chloride.
3. Calc-spar. } { 3. Calcium carbonate.
4. Fluor-spar. } Can be { 4. Calcium fluoride.
5. Apatite. } scratched { 5. Calcium phosphate.
6. Felspar. } by knife or { 6. Potassium and
} file { aluminum silicates.
7. Quartz (rock-crystal).} Cannot { 7. Silica.
8. Topaz. } be scratched { 8. Aluminum
} by { fluosilicate.
9. Corundum (sapphire, } knife { 9. These gems are
ruby). } or { crystallized alumina.
10. Diamond. } file {10. Crystallized carbon.
As a first inquiry into the chemical composition of a mineral, dilute
hydrochloric or sulphuric acid is tried. All _carbonates_ effervesce
when placed in acid or when acid is dropped upon them, while quartz and
all the _silicates_ show no effervescense when so treated.
The table on pages 104-7 contains a brief description of the distinctive
physical features of a number of the very common or important minerals.