Effect of Pleochroism on Gemstone

In addition to its value in gem testing, pleochroism increases the
beauty of some gems. In directions other than along an optic axis, two
different colors emerge from pleochroic stones, but they blend so that
the eye usually sees them as one color, just as the beams of two colored
spot lights on a white surface produce a blended color. In ruby, for
example, both orangey red and violet red continually emerge, but the
eye sees only the blended color, which is more attractive than either
color alone. On the other hand, the color emerging from a direction
parallel to the optic axis will not consist of blended colors. Thus
in many gems, particularly ruby, sapphire, amethyst, kunzite and tourmaline,
there is often a subtle change of color from one part of the stone to
another as the gem is viewed from different directions; for example,
if held toward a strong light, the color seen along the girdle will
differ from that seen through the table. This is explained by the fact
that light transmitted along the direction of single refraction will
be seen as only one color that corresponds to one of the pleochroic
colors. In any other direction, pleochroism will be encountered and
the apparent color of the stone will be the result of the blended colors.

Figure 7 illustrates a still different situation where two colors
are seen from the same direction, because light is totally reflected
from the back facets and thus travels in more than one direction in
the stone. Beam A passes into the stone and is totally reflected from
the end pavilion facet along the length of the stone (in this case,
parallel to the optic axis), and is then reflected out of the stone.
The color displayed will be the one that is common to the direction
of single refraction, if that is the direction imparting the strongest
color to light following it. Beam B, on the other hand, passes in and
out of the stone in the same manner but is always at right angles to
the optic axis; therefore, it displays the blended colors.