Fall 2000 Gems & Gemology - Gemfrance

fluorescent light; for this sample, these colors were dark
yellowish green and brown-orange. In this case, we also
looked at the pleochroism in incandescent light, and were
surprised to find that the stone still showed the same
brown-orange pleochroic color, but the yellow-green was
much brighter (lighter and more saturated). The overall
effect was an oenophile’s delight: In incandescent light, this
tourmaline resembled red Burgundy wine in its typical
green bottle; however, in daylight-equivalent fluorescent
light, the bottle looked empty (figure 20)!
Other gemological properties were as follows: optic
character—uniaxial negative; (Chelsea) color filter reaction—orange
to red; refractive indices—1.620–1.640;
birefringence—0.020; specific gravity (measured hydrostatically)—3.06;
inert to long-wave UV radiation, and
very chalky weak greenish yellow fluorescence to shortwave
UV. A spectrum taken with a handheld spectroscope
in the brown-orange direction revealed a 400–500
nm cutoff, a weak band at 610–630 nm, and weak lines
at 650 and 670 nm. Magnification revealed stringers and
growth tubes, which are typical inclusions in green tourmaline.
At our request, Sam Muhlmeister collected
EDXRF spectra, and found major Mg, Al, Si, and Ca, and
trace Ti, V, Cr, Fe, Zn, Ga, and Sr. Both the chemistry
and the gemological properties are consistent with
uvite, the calcium magnesium tourmaline. Perhaps the
vanadium and chromium contents are responsible for
the shifts in the green pleochroic color that cause this
unusual visual effect.
Although Mr. Boyle purchased this tourmaline in
Sri Lanka, he cautioned us that Sri Lankan dealers now
get their materials from many areas and this stone
might have come from somewhere else. The chemistry
and gemological properties of this stone are in good
agreement with the tourmalines from Umbasara,
Tanzania, which were described in two reports in the
Journal of Gemmology (A. Halvorsen and B. B. Jensen,
“A new colour-change effect,” Vol. 25, No. 5, 1997, pp.
325–330; and Y. Liu et al., “Colour hue change of a gem
tourmaline from Umba Valley, Tanzania,” Vol. 26, No.
6, 1999, pp. 386–396). In those tourmalines, one
pleochroic color is green and the other shifts from
Figure 20. This 2.33 ct tourmaline
seems to shift colors in the brown
pleochroic direction, but in fact it is
the green direction that shifts in
color, as shown here in incandescent
light (left) and daylight-equivalent
fluorescent light (right). Courtesy of
Jay Boyle; photos by Maha Tannous.
Figure 19. Gem-quality cuprian elbaite was reportedly
recovered from this shaft at the Mulungu mine in
Rio Grande do Norte State, Brazil. An electric winch
and steel bucket are used to transport both miners
and rock material. Photo by Brendan Laurs.
orange to “wine red” as the sample thickness increases.
Thus, in the stone we examined, the transmitted
pleochroic brown-orange color would appear as red
reflections (with about twice the optic path length)
when the stone was viewed table-up.
One final caution: According to the literature, tourmalines
can be treated to appear to have an “alexandritelike”
change of color, by painting some facets with red
ink (G. L. Wycoff, “What’s happening in gemcutting,”
American Gemcutter, No. 122, 1997, pp. 3, 26–28). We
checked this stone for inked facets but saw no evidence
of this treatment.
Gem News GEMS & GEMOLOGY Fall 2000 271