What phenomenon does pleochroism refer to?

Pleochroism refers specifically to the ability of a gemstone to display different colors when viewed from different angles. This optical phenomenon occurs in certain minerals that are anisotropic, meaning their properties change based on direction. As light passes through these stones, it interacts differently with the crystal structure depending on the angle of entry, resulting in a range of colors being visible. This characteristic is most commonly observed in materials such as tourmaline and iolite, making it a critical aspect to consider when evaluating colored stones.

The other options describe different phenomena that do not relate to pleochroism. For instance, the emission of light in dark conditions is associated with fluorescence, while displaying a single uniform color pertains to isotropic materials. Furthermore, when a gem absorbs all incoming light, it would appear black, which is unrelated to the concept of varying colors from different viewing angles.