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Color Change phenomenon in Gems @ Jewel Info 4 U
By: NASRINA PARVIN KHAN
Color change is a distinct
optical phenomenon. It helps the
gems to change their color
distinctly under different lighting
conditions, such as incandescent
light to natural light. Alexandrite
is the best example for showing
color change effect. For this reason
usually it referred as "Alexandrite
effect". This phenomenon is also
found in sapphires and garnets
occasionally.
Color change effect first discovered
in the chromium colored variety of
Chrysoberyl called Alexandrite.
Alexandrite is the gem which shows
true color change effect. On the
other hand others gemstones are
called as Alexandrite like.
Gems change their color according to
the nature of the light they are
seen. Alexandrite appears gr4een in
daylight or fluorescent light (blue
in rich) and in incandescent light
(orange to red rich light) it
appears as red.
Chemical composition of the stone is
responsible for this property of the
color change effect. Presence of
chromium+++ ion as a major impurity
in the stones works as coloring
agent. This composition is the cause
of spectacular color change
effecting gems. Rarely vanadium can
also play a part.
Due to their chemistry color change
gems do not absorb colors, such as
red and green. So, these colors
reflected from the gem and we see
the colors.
In daylight, there is little to
absorb blue because of daylight's
richness in blue. So, the gem
reflects blue and appears as green
colored stone in daytime or under
fluorescent lighting source.
Similarly in incandescent light due
to its richness in red, there is
little to absorb red for the gem and
we get the red stone.
Usually the ranges of changing color
are from green to yellow, brownish
red to purple, yellow green to
bluish green.
The strength of color change is
depending on the difference in the
areas of transmission and absorption
in the yellow. The greater
difference shows the stronger color
change.
When color change gems viewed in
light made up of all wavelengths,
the gems transmit blue and red
usually look purple or purple gray.
Alexandrite is a true color change
gem. It appears green to blue green
in daylight and purple to purplish
red in incandescent light. Cr+++ ion
in its composition is responsible
for this effect, which also gives
the rich hues in ruby and emerald.
Alexandrite first found in Tokovaya
deposits, Russia and named after the
Russian Tzar Alexander II at his
coming of age ceremony. It accepted
as the national stone of Russia
because of its red and green colors,
which were also the colors of the
old Russian Imperial.
Alexandrite is also found in Sri
Lanka, Zimbabwe, Burma, Tanzania,
Madagascar, India, and Brazil etc.
Except Alexandrite, there were
garnets and sapphires which also
posses the phenomenon of changing of
color occasionally.
A color change garnet is known as
"Alexandrite garnet" in the field of
gemology. It changes color from
smoky grayish blue (in day light) to
vibrant raspberry red (in
incandescent light) in case of color
change garnet vanadium plays a major
role for its color shift. A little
amount of chromium with a high
concentration of vanadium in its
composition is responsible for the
color change phenomenon.
Color change garnets are rare and
comparatively small. It is found in
Madagascar and East Africa.
Occurrence of color change sapphires
are extra ordinarily rare. They tend
to change their color from yellowish
green to slightly bluish green in
daylight and bluish purple to bluish
violet in incandescent light.
Chromium and vanadium in its
composition play the roles to posses
the uncommon phenomenon. It is found
mainly in the Umba River Valley of
Tanzania.
Rarely we can found a color change
natural stone, which shows a
complete color change. The
completeness of the change of color
is generally revealed by the
percentage. For example we can say
that a specific stone shows an 80 %
color change.
Color change phenomenon is also
depends on the thickness of the
stone with the light. More color
change is visible with the thicker
stone.
Color change factor justify the
value and beauty of the stone. The
particular colors noted in the stone
are very important for these. More
visible color change adds more value
for the stone.
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