Pleochroism: Gems That Change Color, Causes, Examples
Pleochroism is a fascinating optical phenomenon observed in certain gemstones and minerals. It refers to the ability of these materials to exhibit different colors depending on the viewing direction. This variation in color arises from the unequal absorption of light by the crystal structure.
Light entering a pleochroic gem travels along two or three vibration directions within the crystal. Each direction may absorb certain colors of light more strongly than others. As a result, the gem appears to change color depending on the angle at which light strikes it.
Pleochroic gem can show a maximum of three distinct colors depending on the viewing direction.
Pleochroism comes from Greek, with "pleo" meaning "more" and "chroma" meaning "color." So it literally translates to "more color," perfectly capturing the essence of a pleochroic gem's ability to show multiple colors.
Causes of Pleochroism in Gemstones
Pleochroism is a direct result of the following factors:
Crystal Structure: Gems and minerals with a specific crystal structure, where the arrangement of atoms is not uniform in all directions, exhibit pleochroism. Unlike isotropic materials where light travels at the same speed in all directions, pleochroic gems have an anisotropic crystal structure. This means the arrangement of atoms within the crystal is not uniform in all directions. This non-uniformity creates varying densities and electronic environments along different crystallographic axes.
Polarized Light: Light is a wave phenomenon with electric and magnetic fields vibrating perpendicular to each other. In unpolarized light, these vibrations are random in all directions. However, certain materials can polarize light, meaning the vibrations are confined to a single plane. When unpolarized light enters a doubly refractive (birefringent) crystal, it gets split into two rays with mutually perpendicular polarizations that travel at different speeds through the crystal.
Selective Absorption: Different colors of light correspond to different wavelengths. As the two split, polarized light rays travel through the crystal at varying speeds, they interact with the electron structure of the atoms in an uneven way. This is because the electron cloud surrounding each atom can absorb specific wavelengths (colors) of light more readily than others. Due to the non-uniform crystal structure, these absorption strengths differ along different crystallographic directions. So, one polarized ray might lose more red light, while the other loses more blue light, depending on the gem's specific structure. This unequal absorption is what causes the color variation observed in pleochroic gemstones.
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| Pleochroic Tanzanite crystal. Photo: Laszlo Kupi |
Types of Pleochroism
Based on the number of colors exhibited, pleochroic gems can be classified into two categories:
Dichroic
Dichroic, This type refers to gems that display two distinct colors depending on the viewing direction. These gems have a crystal structure that allows for preferential absorption of light along two axes. As a result, you'll see two different colors depending on whether you're looking through one of these dominant axes or not.
Examples of dichroic gems include:
Sapphire: Can exhibit blue and yellow-green colors.
Iolite: Shows violet-blue and yellowish-brown hues.
Emerald: May appear green and slightly bluish-green from different angles.
Trichroic
Trichroic, This type refers to gems that exhibit the maximum of three distinct colors depending on the viewing direction. These gems have a crystal structure with three preferential absorption axes. The unequal absorption of light along these axes creates a more dramatic color change as you rotate the gem.
Examples of trichroic gems include:
Tourmaline: This gemstone can display a wide range of colors depending on the specific variety. Some tourmalines might show green, red, and yellow, while others might exhibit blue, violet, and yellow.
Kyanite: This gem can be blue, green, and yellow depending on the viewing direction.
Alexandrite: This rare gemstone is known for its dramatic color change. It can appear green in daylight and red under incandescent light. However, the color change in alexandrite is not solely due to pleochroism but also a combination of other optical phenomena.
Are all gemstones pleochroic?
No, not all. Isotropic gems (those with a single refractive index) will never display pleochroism. Additionally, even among birefringent (double refractive) gemstones, the difference in light absorption might be too subtle to perceive as distinct colors.
Pleochroism Gemstones Value
The influence of pleochroism on a gem's value depends on the specific gem variety, the intensity and nature of the color change, and how well it's showcased through cutting and setting. Here's a breakdown:
Strong pleochroism can create a fascinating play of color within a gem, adding to its beauty and desirability. This can increase the value for certain gems, particularly those where the color change is dramatic and aesthetically pleasing. For example, some varieties of tourmaline with a strong color shift from green to red or blue to violet are highly sought-after and can command premium prices.
Uneven Color: In some cases, pleochroism can manifest as an undesirable color change. For example, a gem that appears a vibrant green from one angle but a dull brown from another might be less valuable.
Cutting Challenges: While skilled cutters can leverage pleochroism, it can also present challenges. Some pleochroic gems might appear unevenly colored after faceting due to the way light interacts with the crystal structure.
Pleochroism can be a valuable factor influencing a gem's beauty and desirability, potentially affecting its value. However, it's just one aspect to consider alongside other factors like color, clarity, cut, and carat weight.
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| bi-color Sapphires |
The Difference Between Pleochroism, Color Change, and Color Zoning
Pleochroism:
The ability of a gem to display different colors depending on the viewing direction. This arises from the unequal absorption of light by the crystal structure. As light travels through the gem along different axes, certain colors are absorbed more than others, leading to a color change as you rotate the stone.
The ability of a gem to display different colors under different lighting conditions. This is not related to the viewing direction but to the light source itself. For instance, a gemstone might appear green in daylight but change to red under incandescent light. This phenomenon is not related to the viewing direction but to the specific way the gem interacts with different wavelengths of light in various lighting conditions.
The presence of distinct color zones within a single gemstone. These zones can be visible to the naked eye or require magnification. These color variations can be subtle or quite dramatic and may appear as bands, patches, or clouds within the gem. Color zoning arises from variations in the gem's chemical composition during its formation process.
Famous Examples of Pleochroic Gemstones
Here are some famous examples of pleochroic gemstones, showcasing the fascinating variety of color play they exhibit:
Tourmaline: This versatile gem comes in a wide range of colors, and many varieties are pleochroic. Some popular examples include:
- Paraiba Tourmaline: This rare gem displays a captivating neon blue or turquoise color that can shift to greenish-blue or even greenish-yellow depending on the viewing angle.
- Rubellite Tourmaline: This pink to red tourmaline can exhibit a beautiful color change, appearing more reddish from one direction and pinkish-orange from another.
- Watermelon Tourmaline: This unique tourmaline variety features a bi-colored core with a red center that transitions to a green outer layer. The pleochroism can further enhance this color interplay.
Alexandrite: This captivating gemstone is known for its dramatic color change. It appears emerald green in daylight but transforms to a reddish-purple or violet hue under incandescent light. This remarkable pleochroic effect makes Alexandrite a highly prized gemstone.
Sapphire: While not all sapphires exhibit pleochroism, some varieties display a beautiful color shift. For instance, a sapphire might appear a rich blue when viewed down one axis and reveal a subtle greenish tinge when viewed from another direction.
Iolite: This violet-blue gemstone is known for its pleochroism, showing a color change from violet-blue to yellowish-brown or grayish-yellow depending on the viewing direction.
Kyanite: This gemstone is typically trichroic, meaning it exhibits three distinct colors: blue, green, and yellow. The specific color combination and intensity of pleochroism can vary depending on the specimen.
Tanzanite: This captivating gemstone displays a beautiful blue-violet hue with flashes of purple or red. Pleochroism can contribute to the depth and complexity of its color.
These are just a few examples, and the world of pleochroic gems offers a vast array of beauty and color play. When considering a pleochroic gemstone, it's important to understand how the color changes and how the gem is cut to maximize its visual appeal.
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