What changes during the alexandrite color-change effect?

The color-change effect observed in alexandrite is primarily due to the change in hue. This phenomenon occurs because the gemstone exhibits different colors under different lighting conditions, typically appearing green to blue in daylight and red to purplish-red under incandescent light. This dramatic shift is a result of the way alexandrite interacts with light; the presence of chromium ions is responsible for the unique display of color.

When considering the other aspects, such as brightness, saturation, and transparency, while they may be influenced by the light and environment, they do not capture the essence of the distinct color-change effect that alexandrite demonstrates. Brightness refers to the lightness or darkness of a color and saturation describes the intensity of a color, both of which can vary but do not define the characteristic color shift. Transparency is more concerned with the clarity of the stone and is not directly related to the color attributes. Thus, the change in hue is the defining feature of alexandrite's color-change effect.