Pleochroism is the ability of some gems to show different colors, or different shades of the same color, when viewed from different directions. Turn a tanzanite and it shifts between blue, violet and a brownish or reddish tone. Look at iolite one way and it is violet-blue; look another way and it is nearly colorless. The effect comes from how the crystal absorbs light along different directions, and it guides both identification and cutting.
Why pleochroism happens
Pleochroism only occurs in doubly refractive gems. When light enters one of these crystals, it splits into two rays that vibrate in different directions and travel at different speeds. The birefringence page explains this splitting.
In a colored crystal, the atoms responsible for the color sit in an ordered, directional arrangement. Each ray, vibrating in its own direction, can be absorbed differently. One ray might lose more of its red light and look blue, while the other loses more yellow light and looks violet. When you look at the stone from different angles, you see different mixes of these rays, and the color changes.
Singly refractive materials cannot be pleochroic, because light travels the same way in all directions. Diamond, spinel, garnet, glass and opal show no pleochroism. The color in gemstones page covers what causes color in the first place.
Dichroism and trichroism
How many colors a gem can show depends on its optic character.
Uniaxial gems belong to the tetragonal, hexagonal and trigonal crystal systems. They have one direction of single refraction (the optic axis) and can show two pleochroic colors. This is dichroism. Ruby, sapphire, emerald, aquamarine, tourmaline and quartz are uniaxial.
Biaxial gems belong to the orthorhombic, monoclinic and triclinic systems. They can show three pleochroic colors. This is trichroism. Tanzanite, iolite, andalusite, topaz, kunzite and peridot are biaxial. Only two of the three colors are ever visible at once in a dichroscope, so a gemologist turns the stone to see the third.
Pleochroism is often called weak, moderate, strong or very strong depending on how different the colors look. In some gems the effect is so faint that it is hard to see.
The dichroscope
The dichroscope is a small handheld tube that shows two pleochroic colors side by side. The most common type, the calcite dichroscope, contains a piece of optically clear calcite. Calcite has very high birefringence, so it splits the light coming through the gem into two images of the window at the end of the tube. Each image shows light vibrating in one direction. If the gem is pleochroic, the two windows look different in color.
A newer, cheaper type uses two pieces of polarizing film set at right angles. It works on the same principle with a simpler design.
To use a dichroscope:
- Hold the gem close to a bright white light, with the light passing through the stone.
- Hold the dichroscope close to the gem and look through the eyepiece.
- Rotate the dichroscope and turn the gem in several directions.
- Compare the two windows. A difference in color or shade means the stone is pleochroic.
A gem that shows no pleochroism in any direction is either singly refractive or being viewed along an optic axis. The gemological instruments page shows where the dichroscope fits with other tools.
Gems with strong pleochroism
| Gem | Pleochroic colors | Strength |
|---|---|---|
| Tanzanite | Blue, violet, and brownish yellow to reddish (before heating) | Strong trichroism |
| Iolite | Violet-blue, light blue, near colorless to yellowish | Strong trichroism |
| Andalusite | Green, yellowish green, reddish brown | Strong trichroism |
| Kunzite | Pink to violet, near colorless | Strong |
| Tourmaline | Darker and lighter shades of body color | Moderate to strong |
| Ruby | Purplish red and orangy red | Moderate to strong |
| Blue sapphire | Violetish blue and greenish blue | Moderate to strong |
| Emerald | Bluish green and yellowish green | Moderate |
| Aquamarine | Blue and near colorless | Weak to moderate |
| Alexandrite | Green, orange, purplish red | Strong |
Tanzanite
Rough tanzanite from the Merelani Hills is usually brownish before heat treatment. Heating removes the brownish pleochroic color, leaving the blue and violet ones. Even heated stones show a strong shift between blue and violet, which gives tanzanite much of its appeal.
Iolite
Iolite, the gem variety of cordierite, shows violet-blue in one direction and almost no color in another. Viking navigators are sometimes said to have used iolite as a "sunstone" to find the sun on cloudy days. That story is a popular legend; historians debate which mineral, if any, was used.
Andalusite
Andalusite shows green and reddish brown at once in a faceted stone. Cutters often orient it to display both colors together, so the stone flashes a mix of colors instead of one even tone.
Cutting orientation
Pleochroism matters most to the cutter. The color seen through the table of a finished gem depends on how the rough was oriented on the faceting machine.
- In tourmaline, the color along the length of the crystal (the optic axis) is often very dark. Cutters usually place the table parallel to the length to show the lighter, brighter color.
- In ruby and sapphire, the purer, more desirable color is usually seen down the optic axis. Cutters try to orient the table perpendicular to that axis, though the shape of the rough and yield often force compromises.
- In kunzite, the strongest pink is seen along the length of the crystal, so the table is usually cut across it, facing down the long axis.
- In tanzanite, cutters choose between showing more blue or more violet, which affects how the market judges the stone.
A badly oriented gem can show an off color or an unwanted extra hue through the table. When buying pleochroic stones, look at them face-up under different lights, since that is how they will be seen in jewelry.
Pleochroism in identification
Pleochroism helps separate gems that look alike. A red stone with clear dichroism is probably ruby or another doubly refractive gem, while one with none could be spinel, garnet or glass. The specific colors also help: ruby shows purplish red and orangy red, while red tourmaline tends to show two shades of pink to red. Pleochroism works alongside refractive index and other tests, as explained in the guide on how to identify gemstones.
Frequently asked questions
What is the difference between pleochroism and color change?
Pleochroism changes color with viewing direction. Color change, as in alexandrite, changes color with the light source, such as daylight versus incandescent light.
Can a diamond be pleochroic?
No. Diamond is cubic and singly refractive, so it shows no pleochroism.
Why does my tanzanite look more blue in some light and more violet in other light?
Tanzanite's pleochroism combines with how lighting affects its color. Daylight and cool lighting tend to bring out blue, while warm light brings out more violet.
Do I need a dichroscope to see pleochroism?
Strongly pleochroic gems, such as iolite, andalusite and tanzanite, show it to the naked eye as you turn them. A dichroscope shows weaker pleochroism and displays the colors side by side.