In order for a color change in a gemstone to occur there must be two distinctly different light sources available. Color change gemstones such as garnet or alexandrite typically display a bluish color when viewed under daylight (whether natural or artificial) while incandescent light is responsible for the purplish/pinkish/reddish colors.
The color-change phenomenon in gemstones is a result of the “near-equal” transmission of light from the blue-green and red portions of the color spectrum coupled with a strong absorption of light in the yellow portion. This means that the color produced by color-change gems is dependant on the spectral strength and position of the light source. Incandescent light is heavily tilted to the red end of the spectrum and therefore causes color-change gems to appear reddish. Daylight is more equally balanced and since our eyes are most sensitive to bluish green light, the transmission of light is therefore seen on the bluish or greenish side. The strength of the color change is a result of the difference between two light sources and relative to the absorption in the yellow. The greater the difference, the stronger the color change we see.
To witness the color-change phenomenon under ideal conditions, place your color-change gem near the edge of a table in daylight and then using an ordinary lit candle wave the flame in front of the gem. This should result in your color change garnet or alexandrite changing color. Now if you walked into a candlelight room or a room only lit by incandescent light – the entire gem will be the hot purplish-reddish color. Be sure that when viewing the color change that you keep the light sources “pure” (not mixed with the other light type), otherwise you will see a dull effect or no effect at all.