Alexandrite is a variety of the mineral chrysoberyl that exhibits dichroism or a change in color depending on the light source. It can appear green in daylight and red in incandescent light, earning it the nickname of "emerald by day, ruby by night." This phenomenon is caused by trace amounts of chromium in the crystal structure. The best alexandrite exhibits a vivid color change, and rare high-quality stones can be valued above diamonds. Alexandrite is found in pegmatites and ultramafic rocks in India, Sri Lanka, Russia, and Brazil.

1. ALEXANDRITE
Alexandrite is popularly known as
“Cat’s Eye”
A gem variety of crysoberyl
Chemical composition BeAl2O4
Hardness-High-8.5, Third rank in
hardness

2. Crystal system orthorhombic
Occurs as tabular crystals, which are
often twinned.
Iron impurity is usually present in
traces, resulting in shades of yellow,
brown and green.

3. Presence of Cr imparts a dramatic
change in color.
Fluid inclusions and liquid feathers
are common.
Color of common
chrysoberyl/alexandrite ranges from
green, greenishyellow,brown, grayish
white.

4. The color differs depending on the
source of light: This phenomenon is
known as “Alexandrite” or
“Chameleon” effect.
The best example is alexandrite, from
which the effect has been derived its
name.

5. Alexandrite that changes color when
its position of sight is changed, or a
ware like color effect on color tone
effect is seen, which is known as
“Cat’s Eye”.

6. All Alexandrites do not show
spectacular color change.
Cat’s Eye effect also seen in
sillimanite, tourmaline, beryl,
diopside, tremolite, apatite and
moonstone.

7. According to Gemologists, the term “Cat’s
Eye” is exclusively reserved for alexandrite,
a gem variety of crysoberyl(BeAl2O4).
Suitable presence of chromium in
alexandrite shows a significant color
change. However presence of Fe reduces
the quality of color change. However there
is no particular scale to determine the so
called percentage of color change.

8. In the evening light, the color of best
quality alexandrite is red, and in day
light it appears green. Due to this it is
often called”ruby” by night and
“emerald” by day.
This property is called metamerism,
or dichromatism.

9. 0.18% of Cr2O3 in alexandrite may
not show color change.
0.64% of Cr2O3 shows significant
color change.
Price of crystal depends on
percentage of color change, value of
startling color changing alexandrite
goes beyond diamonds.

10. Synthetic corundum grown with chromium
and vanadium impurity showing color
change is available in market.
Like Beryl the crysoberyl is associated
with Be bearing acid igneous rocks.
Common in beryllium pegmatites intending
mica schists and ultramafic rocks. In case
of ultramafic rocks, alexandrite can be
exported because of chromium in
ultramafic rocks. The formation of it is
attributed to desilication and addition of
chromium.

11. Distribution:
Alexandrite has been reported from
Western Ghats south of Kaveri River.
Kumaun and Garwal of Himalayas
Coimbatore-salem region of Tamil
Nadu Shevrai hills of Tamil Nadu
Manikkal of Kerala state.