What type of gem can show pleochroism?

Pleochroism is a phenomenon exhibited by certain gemstones, whereby they display different colors when viewed from different angles. This effect is observed in gemstones that have a crystalline structure that allows light to be absorbed differently in various directions.

Doubly refractive gemstones, or those that exhibit birefringence, have two distinct refractive indices. This unique property enables them to refract light in two different ways and contributes to the visibility of pleochroism. When light enters such a stone, it interacts with the different axes of the crystal, resulting in various color perceptions based on the orientation of the viewer relative to the crystal structure.

In contrast, single refractive gemstones do not exhibit pleochroism because they have only one refractive index and therefore do not split light into different paths. Opaque stones cannot show pleochroism, as they do not allow light to pass through, making the phenomenon impossible to observe. Transparent stones might show pleochroism, but only if they possess the necessary birefringent characteristics; however, the term "transparent" alone does not imply that pleochroism is present, as it could refer to various types of crystals with different optical behaviors.

Thus, the correct answer highlights the characteristics of