4.7 single and double
Beryl 1.575 1.0 varies considerably
Topaz 1.635 4.1 " "
Chrysoberyl 1.765 1.1 " "
Ruby 1.949 5.1 single and double

The tourmaline has a light-refractive index of 1.63, with a heat index
of none, being to heat-rays completely opaque.

The refractive index of gypsum is 1.54, but heat none, being opaque.

The refractive index of amber is 1.51, but heat none, being opaque.

In some of the specimens the gypsum showed a heat-penetration index of
0.001, and amber of 0.056, but mostly not within the third point. In all
cases the heat-penetration and refraction were shown by electric
recorders. These figures are the average of those obtained from tests
made in some cases on several stones of the same kind, and also on
isolated specimens. Not only does the power of the stone to conduct
heat vary in different stones of the same kind or variety, as already
explained, but there is seen a remarkable difference in value, according
to the spot on which the heat is applied, so that on one stone there is
often seen a conductivity varying between 0.15 to 4.70.

This is owing to the differences of expansion due to the temporary
disturbance of its crystalline structure, brought about by the applied
heat. This will be evident when heat is applied on the axes of the
crystal, on their faces, angles, lines of symmetry, etc., etc., each one
of which gives different results, not only as to value in conductivity,
but a result which varies in a curious degree, out of all proportion to
the heat applied. In many cases a slight diminution in applied heat
gives a greater conductivity, whilst in others a slight rise in the
temperature of the heat destroys its conductivity altogether, and
renders the stone quite opaque to heat-rays.

This anomaly is due entirely to the alteration of crystalline structure,
which, in the one case, is so changed by the diminution in heat as to
cause the crystals to be so placed that they become diathermal, or
transparent to heat-rays; whilst, in the other instance, the crystals
which so arrange themselves as to be diathermal are, by a slightly
increased temperature, somewhat displaced, and reflect, or otherwise
oppose the direct passage of heat-rays, which, at the lower temperature,
obtained free passage.

Thus certain stones becom