Which property is most responsible for a gem having two refractive indices?

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

Answer: (B)

Two refractive indices appear because of birefringence, which occurs when a material is optically anisotropic—the speed of light depends on the direction and polarization as it travels through the crystal. The crystal structure dictates this directional dependence: in anisotropic crystals, the lattice arrangement makes the refractive index differ along different axes, so light splits into two rays with different indices (ordinary and extraordinary). In isotropic materials with a uniform structure, light sees the same index in all directions, so there is only one refractive index. That’s why crystal structure is the factor that controls whether a gem can have two refractive indices. For context, calcite shows strong birefringence, quartz is uniaxial with two indices, while diamond’s cubic structure makes it effectively isotropic with a single index. Atomic weight, color, and density do not determine this directional optical behavior.