aluminum borosilicate with changeable composition
7.5 Suitable for all jewelry uses.
structure: trigonal, also considered hexagonal for reasons
illustrated in the picture below.
Pleochroism: moderate to very
(Text and Photos by
Tourmaline has become an increasingly
popular gemstone because it comes in such a wide range of beautiful
colors. This range of colors is caused by its complex
chemical make-up and its ability to attract and hold metal ions
during crystal formation. Vanadium 5+and
chromium 3+ can
result in a pure rich green. If a crystal attracts iron and
iron oxide it can result in a stunning sapphire blue.
The colors in a
tourmaline can tell the story of its formation. When the pink
and white bi-colored stone pictured below was formed, the manganese
necessary for the pink was not present at the beginning of the crystallization
cushion pictured below has even a stranger growth history. The
iron causing the attractive greenish-blue color when the crystal
began to form was altered at the end of the growth process, giving
the outer layers an unattractive khaki color.The attractive color was
The medium green round
pictured below underwent a similar process and the resulting piece of
rough was so unattractive that I put off cutting it for an entire
form within pegmatites, which are coarse grained granites which have
intruded into pre-existing rock formations under great
pressure. As the melt cools, various types of crystals
begin to form in pockets. If the pockets are large enough and
the cooling rate is slow enough, large crystals are able to form.
Brazil is famous for its huge pegmatite veins with pockets large
enough to allow the growth of huge crystals. Fred Rynerson, who
mined tourmaline in the early 1900's in southern California,
described pockets measuring as much as 55 feet long and 40 feet wide
in his classic book "GEMS & GOLD". Tourmalines
have also been found in contact-metamorphosed limestones, schists and
gneisses but they are seldom of gem quality.
Tourmaline is much more dynamic than other gemstones because of
discoveries of new types and colors. In the late 1980's,
tourmalines containing copper were discovered in the Sao Jose Batalha
area of Paraiba, Brazil and the colors of these stones were
spectacular. As the Paraiba deposits began to dwindle,
beautiful blue and blue/green gems were found in Namibia, Nigeria and
color-change stones containing bismuth and/or copper were found in
Mozambique in 2004 also.
In the last couple of
years yellow and coppery-pink tourmalines have been found during the
search for "Paraiba-type" gems in Mozambique.
Recently I cut
the red round brilliant pictured below from a red tourmaline rough
that looked so much like a chrome-pyrope garnet that I immediately
tested its specific gravity, pleochroism and absorption spectra prior
to cutting. Since it also shows a fairly strong reaction to the
I think more testing of
this stone could be interesting.
seemingly endless color range of tourmaline will insure continued
interest in this 'rainbow gem.'
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