Birefringence

A calcite crystal laid upon a paper with some letters showing the double refraction

Birefringence (or double refraction) happens when light is passed through a special material and it splits into two different rays of light. Calcite crystals and boron nitride are two good examples of materials that cause birefringence. Just as a prism splits light into a large number of colors, birefringence splits light into two images of the same color. You see this as a "double image".