II. Physical Properties Dependent on Heat.
forms of radiant energy, heat and light, are so closely related that we
might expect their manifestations in relation to the precious stones to
be much alike. This is so, for heat rays may be reflected, refracted,
or absorbed, as may light rays. Effects akin to polarisation may be
observed too, but none of these effects are of the same importance in
the general consideration of gem stones as are the results of the
action of light. A few facts may, however, be briefly stated as of
easily passes through Fluor Spar, hence it is said to be diathermanous,
while Tourmaline, Gypsum and Amber are almost opaque to heat rays.
conductivity of heat is found to vary in different minerals and in
different directions in relation to the crystal axes ; the coefficient
of expansion is different in these several directions in many cases,
and this brings about changes in the optical characters under these
conditions. On the whole the precious stones are good conductors of
heat as mineral substances go, and hence it is stated in Mr. H.
Spencer's translation of Max Bauer's " Precious Stones," that this may
sometimes be used as a means of distinguishing between a true and a
false gem ;