What does the term "double refraction" refer to?

The term "double refraction" refers specifically to the phenomenon where light enters a material and is split into two distinct rays, each following a different path. This occurs in certain minerals, such as calcite and some other crystals, which have an anisotropic structure. When unpolarized light hits these materials, it is refracted into two separate rays with different velocities and indices of refraction. This property can be observed as a distinctive double image when looking through such a crystal.

In the context of gemstones, this double refraction can be an important identifying characteristic, as it affects how light interacts with the material and thus how the stone looks to the observer. Understanding this concept is crucial in gemology, as it helps to differentiate between various types of gemstones based on their optical properties.