What Causes Pleochroism?
A gem’s crystalline structure largely determines whether it’s non-pleochroic, dichroic, or trichroic. That structure affects how light passes through the material. Light may be polarized and absorbed differently along different paths, which may affect the colors we see.
When light passes from air into an object like a gemstone, it slows down. This is called refraction. A gem’s refractive index (RI) is the ratio of the speed of light in air to the speed of light through the gem. We see the light refracted through the gem as color.
If nothing else restricts the movement of light, the stone is called isotropic. In isotropic materials, light passes in every direction at the same speed. They don’t polarize the light. This occurs in amorphous materials like amber (materials without a crystal structure) as well as crystalline materials with a single RI. Gemstones such as diamonds and garnets belong to the isometric or cubic crystal system. These gems have a single RI, so no matter the viewing angle, they only show one color. They’re non-pleochroic.
Anisotropic or Birefringent Gems
The gems that belong to the other five crystal systems do polarize light. When this occurs, light vibrates in two planes at right angles to each other. As a result, light moves at different speeds in different directions in these gems. This means they actually have more than one RI. Furthermore, since polarized light is absorbed differently each direction it takes, each direction of light or “beam” has a different color. These gems are called anisotropic or birefringent. They may show two to three different colors depending on the viewing angle.
The gems in the hexagonal and tetragonal crystal systems have one optic axis (one direction that doesn’t polarize light) but two RIs, so they refract light at two different speeds. These are called uniaxial gems. These gems may show up to two colors. Sapphire and emeralds are well-known examples of uniaxial gems.
The gems in the orthorhombic, monoclinic, and triclinic crystal systems have two optic axes but three RIs, so they refract light at three different speeds. These are called biaxial gems. These gems may show up to three colors. Andalusite, cordierite, and tanzanite are well-known examples of biaxial gems.
This tanzanite crystal displays trichroism: light purple from the front face; gray-blue from the side, and reddish brown along its vertical axis. 29.72 cts, 2.9 x 1.4 x 0.8 cm, Merelani Hills, Lelatema Mts, Arusha Region, Tanzania. © Rob Lavinsky, www.iRocks.com. Used with permission.
Are All Birefringent Gems Pleochroic?
Colorless gems may not show pleochroism, even if they’re uniaxial or biaxial.
Not all birefringent colored gemstones show pleochroism. Instead, they may only show a single color regardless of viewing angle. The difference in absorption between two or more directions of light may be too slight to perceive as distinct colors. Furthermore, some biaxial gems may only show two pleochroic colors, not three.
Are All Isotropic Gems Non-Pleochroic?
Isotropic (singly refractive) gems never show pleochroism. However, some singly refractive gems may have anomalous birefringence, which may create a pleochroic-like color effect. Isometric gems like diamonds and garnets may show these anomalous colors, but this is typically due to crystal strain, not pleochroism.
What’s the Difference Between Pleochroism, Color Change, and Color Zoning?
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. (Alexandrite shows both color change and pleochroism).
Pleochroic Colors 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 structure of an unknown gemstone. In conjunction with an analysis of its other properties, this can help classify it.
The mineral supergroup epidote contains many gem species. Some display pleochroism, some don’t (even though they all have the same monoclinic crystal structure). Some also show specific pleochroic colors. However, sometimes these stones are too dark to show their colors easily. A strong backlight helps illuminate this epidote crystal’s “root beer” and dark olive-green pleochroic colors. 3.3 x 1.4 x 1.0 cm, Tormiq Valley, Haramosh Mts., Skardu District, Baltistan, Northern Areas, Pakistan. © Rob Lavinsky, www.iRocks.com. Used with permission.
How Can You Observe Pleochroism?
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.
Was the “Viking Compass” a Pleochroic Stone?
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).