As
the property of hardness is of great value in the case of precious
stones, those kinds which are scratched by quartz, and which,
consequently, are below 7 degrees of hardness, are ranked as half-hard,
or " demi-dures." Stones scratched by a knife are below 5 degrees.
There
are two remarks as to degrees of hardness which it is proper to
introduce in this place. Firstly, the degree of hardness of a crystal
or a cut stone varies, generally, however, within narrow limits, on
different faces and in different directions. Secondly, the usually
accepted scale of hardness is one having very different values for the
different intervals. Unlike the degrees of the thermometer where the
interval between one degree and the next above it or below it has the
same value, whatever part of the scale be chosen for comparison, the
degrees of hardness on Mohs' scale show extraordinary divergences.
Specific Gravity.—The
most generally applicable of all modes of discriminating precious
stones from one another is to ascertain their specific gravity—that is,
the relative weights of equal bulks— the weight of a bulk or volume of
distilled water (commonly taken at 60° F. or 15°.6 C.) being employed
as the unit with which all the others are compared. There are three
modes of ascertaining the specific gravity of a stone : (1) By placing
it in heavy liquids of known specific gravity, and noting the position
which it takes up. (2) By weighing it in air, and then in water (or
other liquid), and
