Doubly refractive gems have two refractive indices because of the way light interacts with their crystal structures.

• A. Atomic Weight
• B. Density
• C. Color
• D. Crystal Structures

Answer: (D)

Doubly refractive behavior comes from anisotropy in the crystal structure. In crystals, the arrangement of atoms and the way the lattice interacts with light varies with direction and polarization. Because of that, light propagating in certain directions experiences different speeds and, therefore, different refractive indices. This splits a beam into two rays—the ordinary ray with one index and the extraordinary ray with another—so you see double refraction. In isotropic materials, the structure looks the same from all directions, giving just one refractive index.

Atomic weight, density, or color aren’t what cause this effect. Atomic weight and density don’t by themselves create direction-dependent optical properties, and color relates to which wavelengths are absorbed or transmitted rather than to having two refractive indices. The splitting of light is inherently tied to how the crystal lattice is arranged—its crystal structure—so that is the best explanation. For that reason, crystal structures are the reason behind double refraction in gems.