Color
Change in Alexandrite
Alexandrite
is a trichroic gemstone which may
absorb and reflect light differently
in each of its three optical directions.
The color change phenomena is a
result of the presence of chromium
+3 ions and the way they are absorbed
and reflected. In alexandrite, the
band is at 580nm. When the light
is balanced (daylight), the stone
will be green but when the light
source is reddish (incandescent),
the stone appears red.
Alexandrite
or Chrysoberyl
In
practice, chrysoberyl and alexandrite
may be difficult to differentiate.
Since stones with a weak change
may be called alexandrite and stones
with a faint change are called chrysoberyl,
what is the difference? Where is
the borderline?
Since
the origin of the color change in
alexandrite is due to the presence
of chromium while the color of yellow
or brown chrysoberyl is due to the
presence of iron, a close examination
of the spectroscopy will reveal
the differences as both chromium
and iron display characteristic
spectrums. The yellow variety shows
a broad band centered at about 4450
A in the blue violet while the alexandrite
variety shows a chromium spectrum.
The presence of chromium lines in
the red orange area of the spectrum
are diagnostic for alexandrite.
Fluorescence
helps us to differentiate. Chrysoberyl
owes its yellowish color to iron
and usually shows no fluorescence.
The red fluorescence of alexandrite
can be observed using the “crossed
filter” method and it has been inferred
that in border line cases the presence
of a red glow and a faint chromium
spectrum absorption spectrum would
prove the stone to be alexandrite.
