What does double refraction in a gem's structure do?

Double refraction, also known as birefringence, occurs in certain gemstones with a crystalline structure that is not isotropic, meaning the properties of the material vary depending on the direction of the light passing through it. When light enters a gem exhibiting double refraction, it is split into two rays that move at different speeds and travel in different directions. This creates two distinct images of an object viewed through the gemstone.

This phenomenon is particularly noticeable in certain minerals such as calcite and quartz. The difference in velocities of the rays results from variations in the gem's refractive index for different crystallographic orientations. As a result, the unique optical effect can be harnessed to enhance certain visual characteristics of the gemstone, such as brilliance and depth of color.

The other options do not accurately describe the nature of double refraction. Specifically, splitting light into three rays is characteristic of other optical phenomena but not double refraction, while the notion of two rays traveling in the same direction contradicts the essence of birefringence. Moreover, although double refraction can influence aspects of a gemstone's appearance, it does not directly affect its color.