14 PRECIOUS STONES
grain, is conchoidal. In some it is even, and in a few, uneven.
The
optical qualities of precious stones when cut and polished are
various. Among them is the power of reflection; that is, when a ray of
light reaches the surface of one, part of it passes through, and part
is thrown back or reflected. This power to reflect light is possessed
by the various gems in different degrees, and the amount of light
reflected varies not only with the stone, but increases in proportion
to the obliquity of the ray as it falls upon it, according to the
stone. The great brilliancy of the diamond is due to the fact that a
ray striking an inner facet at a greater angle than 24 ° 13' is totally
reflected. It is for this reason that precision of cutting is so
necessary in order to secure a maximum of the reflective power of the
gem.
That
part of an oblique, impinging ray which enters the stone does not pass
straight on and through, but, coming into the domain of another
government, is bent or refracted, and the extent of this refraction
varies with the different gems. Again the diamond is superior to all
others, its refractive power being very great, and, in consequence, its
magnifying power also, which is said to compare with that of
plate-glass in the ratio of 8 to 3.
Most
precious stones are doubly refractive. This means that one part of the
ray which enters the stone is bent or refracted at one angle, and the
other diverges to a greater or less degree according to the stone.
These stones are dichroic, and when examined by means of the
dichroiscope, show two images of different colors. This phenomenon is
not absolute, however, as there is a direction, called the optic axis,
in which the light can be transmitted through a doubly refractive gem
without being divided, in which case it acts simply as an ordinary
medium of single refraction. On the other hand, diamonds, which are
singly refractive, have been known to be doubly refractive, owing to
some dis-
