Color Change phenomenon in Gems

by NASRINA PARVIN KHAN

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.

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.