Gems that show two or three colors when viewed from different angles are said to be pleochroic (showing “more colors”). This quality is called pleochroism and can be found quite spectacularly in gemstones such as cordierites, andalusites, and emeralds and more subtly in peridots and aquamarines.
A mineral’s crystalline structure contributes to its pleochroism and whether it’s dichroic (shows two colors) or trichroic (shows three colors). That structure affects the speed at which light is refracted as it passes through the material. The refracted light is the color we see.
Amorphous materials without crystal structure, such as amber, show only one color. Gems like diamonds with an isometric or cubic crystal structure also show one color because the axes of the cubic crystals are identical in length, so refraction is not affected by how the light strikes the gem.
Emeralds, on the other hand, have hexagonal crystal structures with axes of two different lengths. The light that strikes an emerald can be refracted at two different speeds depending on which axis it moves along. Emeralds may thus show two colors.
Some gems have crystal structures with axes of three different lengths. Cordierite and andalusite, with their orthorhombic structures, may show three different colors.
Aside from eye-catching beauty, pleochroism is useful for identifying gems. The number of colors visible from different angles can identify the possible crystalline structure of the gemstone and help classify it. While pleochroism is sometimes appreciable to the naked eye, at other times the differences in color are slight. Gemologists use tools such as a polariscope and dichroscope to better detect pleochroism.
Vikings may have also used pleochroism as a navigational aid. The “Viking Compass” may have used the pleochroic properties of iolite, a variety of cordierite, to detect the sun’s position even on an overcast day or when it was low in the Arctic sky. These “sunstones” as the Vikings called them (not to be confused with the gems called sunstones today) could “could see where the sun was in heaven” and were valuable possessions. One theory suggests that since an iolite stone would have its maximum color change when faced against the direction of the hidden sun, this could have been used by the Vikings to get a directional bearing.