Another type of Phenomenal Gems is the Color-Change Gemstone. These are gemstones that display different colors according to changes in lighting condition, much as a Chameleon monitor lizard, which changes the color of its scale to camouflage itself against other predators to enable it to blend in to the environment.
Now the chameleon does not have the innate ability to change colors according to it’s whims and fancies. External factors such as temperature, danger and emotions of the chameleon are responsible for this. It has a major distribution of melanophores in it’s body which are responsible for the change in color. Since the chameleon’s temperature is controlled from outside its body, its reaction to hot and cold temperatures, leads to a change in the skin color. In addition fear, territorial instincts and danger may make them change their color. All of this is involuntary.
The term Color-Change in gemology is generally used for gemstones that exhibit a distinct change of color hue when the gemstone is exposed to different lighting illumination. Thus, an Alexandrite may appear green under the cool natural daylight (or fluorescent lighting) while under incandescent light (eg halogen lighting, yellow torch light or chandelier lighting) it appears red. This type of Alexandrite is adroitly known as Emerald By Day & Ruby By Night.
The cause of its Color-Change nature is due to the selective absorption of spectral colors within the stone. These Color-Change gemstones have two approximately equal sized transmission windows. A red gemstone appears red because it absorbs all frequencies of light except red. A gemstone that absorbs all frequencies except for blue and red light will appear blue when the light is rich in blue wavelengths (e.g., fluorescent light) and red when the light is rich in red wavelengths (e.g., incandescent lighting).
Apart from Alexandrite other gemstones that may exhibit Color-Change phenomenal includes spinel, garnet and corundum (rubies and sapphires).
Color-Change gemstones are a collector’s item and can be expensive.
Ref : http://www.gemselect.com, http://www.atggems.com, http://gemstonesgarnet.com, http://mineralsciences.si.edu, http://www.alexandrite.net, http://www.independentjewellers.net, http://www.grandgemstones.com, http://image1.gemselect.com, http://www.nataliavalenti.com