Why do some gems show a color change?

Color change gems have two approximately equal sized transmission windows. A blue gemstone appears blue because it absorbs all frequencies of light except for blue. A gemstone that absorbs both blue and red light will appear blue when the light is rich in blue wavelengths (e.g.,sunlight or fluorescent light) and red or purple when the light is rich in red wavelengths (e.g., incandescent light).

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