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Pleochroism
is an optical phenomenon where due to double
refraction of light by a colored gem or
crystal, the light is divided into two paths
which are polarized at a 90° angle to each
other. As the divided light follows different
paths within the stone and are traveling
at different speeds, they may have the result
of differential selective absorption, thus
when they leave the crystal they have different
colors, making the stone seem to be of different
colors.
Some
stones show two colors or shades and are
called dichroic, some show three and are
called pleochroic (or trichroic). Gems are sometimes cut
and set either to display pleochroism or
to hide it, depending on the colors and
their attractiveness.
Dichroic
gems generally always have hexagonal, trigonal, or tetragonal
crystalography. Pleochroic gems generally always have
orthorhombic, monoclinic, or triclinic crystalography.
Isotropic gems are never pleochroic.
Pleochroism
is an extremely useful tool in mineralogy
for mineral identification, since minerals
that are otherwise very similar often have
very different pleochroic color schemes.
Some
gems displaying pleochroism:
Actinolite:
yellow, dark green
Alexandrite:
(strong) deep red, orange yellow, green
Andalusite:
(strong) olive green, brick red; also light red, yellow,
pale yellow
Chondrodite:
yellowish brown, reddish brown, dark red, brownish yellow
Chrysoberyl:
(strong) yellow, brown (also see Alexandrite above)
Diaspore:
(strong) pale green, rose red; also colorless, violet-blue,
dark red
Enstatite:
(strong) pink to green
Hiddenite:
(strong) green, blue-green, pale green, colorless
Iolite:
(strong) colorless, violet or blue; also gray, pale yellow
Kornerupine:
(strong) green, yellowish brown; also brown, greenish
yellow
Kunzite:
(strong) pink, purple-violet, colorless
Kyanite:
(strong) violet-blue, colorless, cobalt-blue; also blueish green
and green
Sillimanite:
(strong) pale yellow, green; also pale to dark brown,
blue, violet-blue
Sinhalite:
(strong) pale brown, greenish brown, dark brown
Sphene:
(moderate to strong) pale yellow, greenish yellow, orange-brown
Staurolite:
(strong) colorless, yellow, red, golden yellow
Tanzanite:
(strong) purple, blue; also yellow or green (Zoisite)
Zoisite:
(strong) yellow, green
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