Because of its great color range, which includes pink, green, blue,
yellow, brown, and black in many different shades and combinations of
shades, tourmaline is one of the most popular of the colored gemstones.
Tourmaline with a color near emerald green is particularly popular.
Chemically, tourmaline is a very complex borosilicate, and its color is
determined by the various elements present in it. Tourmaline crystals
having sodium, lithium, or potassium are either colorless, red, or
green; those having iron are blue, blue-green, or black; and those
having magnesium are colorless, yellow-brown, or blackish brown.
Some crystals of tourmaline are of two colors, and stones of mixed
colors, such as pink and green, can be cut from these. The color mixing
may show as zoning with the core color of the crystal overlaid by
another color and perhaps even additional layers of other colors. Zoned
crystals with a core of deep pink covered by a layer of green have been
called “watermelon tourmaline.” Because its refractive index of about
1.6 is too low to give it marked brilliance, and its color dispersion is
too low to give it fire, the tourmaline relies almost solely on the
beauty of its color for its rank in popularity.
Although tourmaline has a low refractive index and low dispersion, it
exhibits remarkable dichroism. In other words, it can present different
tints to the viewer depending on the direction that the light is
traveling through the crystal. When viewed down the long, or vertical,
axis of the crystal, the color of tourmaline is much stronger than when
viewed from the side. This means that if the crystal is dark the cutter
will have to cut the stone with the flat part, or table, parallel to the
long axis of the crystal. The color of the gemstone then will be
lightened when viewed from its table, since this is the direction of
lighter color. Similarly, the table of a lighter colored crystal can be
cut perpendicular to the long axis in order to produce a deeper colored
gem.
[Illustration: Green seems to be the best known commercial color of
tourmaline, but this extremely variable gem species exhibits many
subtle shades of color, as shown here. At upper left, a 104-carat
stone from Mozambique; at upper right, a 173-carat stone from
Mozambique; at lower left, a 111-carat stone from Manchuria; and a
35-carat stone from Brazil. (Actual size.)]
Some tourmaline crystals contain threadlike tubes or inclusions of
microscopic size running parallel to its length. When cut as cabochons,
such crystals give a good “cat’s-eye” effect.
Tourmaline has no distinct cleavage and has a hardness somewhat above 7,
and these characteristics make the stone sufficiently resistant to
normal shock and wear so that it is highly satisfactory for use in
jewelry.
Noted deposits of tourmaline are located in the Ural Mountains of
Russia, Ceylon, Burma, South-West Africa, Madagascar, Brazil, Maine, and
California. Crystals from each of these localities seem to have their
own color specialties. The deposits in San Diego County, Calif., are
unique in that all colors except brown are found there. In the early
1900’s pink and red tourmaline was shipped from there to China for
carving, but this thriving trade stopped with the end of Chinese
imperial reign. The tourmaline deposits at Paris, Auburn, and Hebron,
Maine, have furnished a number of excellent gems, especially of blue and
green colors.
VARIETIES
Achroite: Colorless
Indicolite: Blue
Dravite: Brown
Schorl: Black
Rubellite: Pink