<![CDATA[Usambara effect suggested by the Mineralogical Museum of Oslo
after Usambara Mountains of Tanzania, where Norwegian geologist Asbjorn Halvorsen collected chromiferous green tourmaline which displayed strong color change effect. The Usambara effect describes a color change dependant not on the type of illumination as with the Alexandrite effect, but rather on a change of path length of light through the gemstone in a single orientation.
With Alexandrite Effect
, if the illumination is stronger in the red wavelengths as with candle light, red becomes the perceived color. In daylight which is stronger in the green wavelengths to which the eye is much more sensitive, the perceived color is green. With Usambara Effect, once the thickness of the gemstone reaches a critical point, dependant on concentration of the elements chromium and vanadium, the perceived color of the transmitted light shifts from green to red. Which explains how Usambara effect was discovered in the first place: gem quality tourmalines from the Umba Valley (Tanzania), each green by transmitted light, when placed on top of one another, showed yellow color, succeeded by orange and red as the thickness of the gemstone pile increased.