Chrysoberyl is the third hardest mineral on the planet coming in after diamond and corundum. One of the three varieties of chrysoberyl is called alexandrite and demonstrates the labeled alexandrite effect. This is when the gem appears to change colour from a blue green to a red when moved from daylight to artificial incandescent light. This is because of all alexandrite materials having a transmission maxima in the blue region and a transmission minima in the red region. Different spectral qualities of light stimulates these transmission lines differently producing the different colours.
A very similar effect has recently been observed in the petals of certain purple flowers. In sunlight they are distinctly purple but in bulb light they take on a reddish tinge and become more bluish and turn violet in white LED light. To properly understand these colour changes an interesting method was used to make sure the researchers were certain of the change. An upgraded colour graph was designed, where as the original is a triangle having green, red and blue as the vertices, the new shape is a circle passing through red, yellow, green, cyan blue and magenta equally. Assuming there were no primary colours made the plotting of the colour change easier and a conclusion was more rapidly reached. It turns out these petals do have some alexandrite effects but also the light response of the human eye helps the change seem more defined. A combination of physics and perception is what causes this interesting quality in these plants.