Pleochroic gems show two or three colors when viewed from different angles. Pleochroism means “more colors.” This property occurs quite spectacularly in gemstones such as andalusite, cordierite, emerald, and tanzanite. In gems such as peridot and aquamarine, it occurs more subtly. Dichroic gems show two colors. Trichroic gems show three colors.
When viewed on its A axis, this tanzanite crystal shows a blue color. On its B axis, purple. (The base is too heavily included to show C axis color). “Zoisite (Var: Tanzanite),” 35.2 cts, Merelani Hills (Mererani), Lelatema Mts, Arusha Region, Tanzania. © Rob Lavinsky, www.iRocks.com. Used with permission.
What Causes Pleochroism?
A mineral’s crystalline structure contributes to its pleochroism and whether it’s dichroic or trichroic. That structure affects the speed at which light is refracted as it passes through the material. We see the refracted light as color.
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. Since the axes of the cubic crystal have identical lengths, how the light strikes the gem has no effect on color.
On the other hand, emeralds have hexagonal crystal structures, with axes of two different lengths. This means an emerald can refract light at two different speeds, depending on which axis it moves along. Thus, emeralds may show two colors.
Some gems have crystal structures with axes of three different lengths. For example, cordierite and andalusite, with their orthorhombic structures, may display three different colors.
Don’t confuse pleochroism with color change or color zoning. Color change gems change color under different types of light, such as daylight, incandescent, fluorescent, etc. Color zoning means a gem has separate areas of color visible on the same axis. (Alexandrite shows both color change and pleochroism).
Pleochroism and Gemstone Identification
Aside from eye-catching beauty, observing pleochroism can help with gemstone identification. The number of colors visible from different angles may indicate the possible crystalline structures of an unknown gemstone. In conjunction with the analysis of other its properties, this can help classify it.
While you can perceive pleochroism with the naked eye in some cases, at other times pleochroic gems show very slight color differences. Gemologists use tools such as a polariscope and dichroscope to better detect pleochroism.
Pleochroism and the “Viking Compass”
Scholars have tried to identify the legendary “Viking Compass,” a stone that “could see where the Sun was in Heaven.” Even on an overcast day or when the Sun was low in the Arctic sky, Viking navigators used this stone to detect the Sun’s position. The Vikings also called this a “sun stone.” (However, this isn’t what gemologists now refer to as sunstone).
One theory identifies this gemstone as iolite, a variety of cordierite. Since an iolite would show its maximum alternate color when faced against the direction of the hidden sun, the Vikings could have used this to get a directional bearing.
An iolite crystal from its blue direction, yellow and blue direction, and yellow/brown direction. Photos by David Abercrombie. Licensed under CC By-SA 2.0. (Slide show created to highlight pleochroic properties).