Gemstones that can change color in different lighting conditions are quite fascinating. This unique phenomenon is called pleochroism, derived from the Greek words pleo (more) and chroos (color). In this article, we’ll explore the science behind color-change gemstones and provide an overview of the most common pleochroic gem varieties.
What Causes Gemstones to Change Color?
Pleochroism occurs due to the molecular structure and optical properties of certain gemstones. More specifically, it is caused by variable absorption of light rays as they pass through the crystal lattice of the gem. The way the gemstone absorbs light differs depending on the polarization plane of the light waves.
When non-polarized white light enters a pleochroic gem, the gem selectively absorbs certain wavelengths and transmits others. As the light passes through the stone, some wavelengths are absorbed more strongly in one direction than another. This gives rise to different spectral colors along different optical directions. So as the gem is rotated, the path of light changes and a different set of wavelengths are absorbed or transmitted, resulting in an apparent color change.
Types of Pleochroic Gemstones
Many gem varieties can display pleochroism to some degree. However, there are a few gems that are highly prized specifically for their dramatic color change abilities:
Alexandrite
Alexandrite is one of the most renowned and valuable pleochroic gems. It can display a stunning color change from bluish green in daylight to purplish red under incandescent light. Russia’s Ural Mountains and Sri Lanka are the main sources for natural alexandrite. Synthetic versions are also available.
Andalusite
Andalusite is strongly dichroic, meaning it shows two distinct colors. Along one direction it appears yellow, olive, or brownish green. Perpendicular to that it can display reddish, pinkish, or orangey hues. The best andalusites are found in Brazil and Sri Lanka.
Iolite
Also known as cordierite, iolite is usually violetish blue but can also shift to yellowish gray depending on the viewing angle. The honey color seen down the principal axis gives it the alternate name of dichroite or water sapphire. Significant iolite deposits have been found in India and Sri Lanka.
Tanzanite
The trademark violetish blue color of tanzanite is strongly pleochroic. The gem displays blue, violet, and burgundy-red pleochroism. Tanzanite is only found in one small area near the base of Mount Kilimanjaro in Tanzania.
Garnet
Some garnet varieties like hessonite and demantoid exhibit a moderate amount of pleochroism. Hessonite can appear yellowish green or orange-brown. While demantoid garnet is greenish yellow along one axis and yellowish green in the perpendicular direction.
Tourmaline
Many tourmalines display pleochroism to some extent. Rubellite tourmaline can range from purplish red to pink. While indicolite tourmaline shifts from light to dark blue. Multicolor tourmaline exhibits the greatest color range based on the viewing angle.
Kyanite
Kyanite is strongly dichroic in trichroic varieties like the vibrant orange, blue, and green kyanite blades found in Tanzania and Kenya. The color differences are striking when viewing down the length of the blade compared to across the width.
Sapphire
Some sapphires show pleochroic behavior, particularly in orange and yellow hues. Blue sapphires can also appear more violet or greenish depending on the orientation. The most prominent color change is seen in alexandrite-effect sapphires.
Spinel
Red spinels often appear more orange when viewed down one optical direction compared to the other. Rare blue spinels can shift between grayish and greenish blue pleochroic colors. The most famous spinels are found in Vietnam and Myanmar.
Zircon
Many zircons display strong pleochroism. Blue zircon can change from sky blue to greenish or colorless. Yellow to brown zircons shift from darker honey-brown to lighter yellow. The most significant zircon sources are Australia, Cambodia, Sri Lanka, and Tanzania.
Moonstone
While best known for adularescence, some moonstones also show pleochroism. The body color can range from colorless to various shades of yellow, orange, pink, and brown depending on the orientation. Prime moonstones hail from Sri Lanka and India.
Hiddenite
Hiddenite is a green variety of spodumene that can display yellowish green to bluish green pleochroism. The best hiddenites are found at the original discovery locale at Hiddenite, North Carolina.
Chrysoberyl
In ordinary light, chrysoberyl is typically yellowish green. But under polarized light, the gem can shift to green, yellow, or brown. The most spectacular variety is alexandrite. Other notable sources for chrysoberyl include Sri Lanka, Brazil, and Tanzania.
Apatite
Apatite occasionally displays pleochroism. Blue apatite can change from violetish blue to greenish or yellowish blue. While green apatite shifts from yellow-green to bluish green. Significant apatite sources include Brazil, Mexico, and Kenya.
What Causes the Strength of Color Change?
The visibility of pleochroism depends on several optical factors:
- Dichroism vs. trichroism – Dichroic gems show two colors while trichroic gems show three colors.
- Absorption spectrum – The wavelengths of light absorbed along each axis affects the pleochroic colors.
- Diffraction – How much the light rays bend as they pass through the crystal lattice.
- Dispersion – How much the white light splits into spectral colors.
- Birefringence – The degree of light ray separation as they pass through the gem.
The strongest pleochroism occurs when there is significant variation in light absorption between the optical directions combined with strong dispersion effects. This leads to dramatic color changes like those seen in alexandrite and kyanite.
How to Observe Pleochroism
To view the color change of pleochroic gems:
- Observe the stone while rotating it or changing its orientation under a light source.
- View through the crown faceting while tilting the stone from side to side.
- Look down the pavilion of the gem perpendicular to the table facet.
- Shine light through the side of a cabochon gem.
- Use a dichroscope which has two fixed perpendicular viewing ports.
- View under crossed polarizing filters which isolate particular optical directions.
The pleochroic effect is most obvious under controlled lighting with monochromatic or polarized light. But it can also be spotted when moving gems between indoor lighting and daylight conditions.
How Gem Cutters Take Advantage of Pleochroism
Skillful gem cutters orient pleochroic gemstones to maximize the visible color change:
- Table facets are oriented to show the most desirable primary color.
- Pavilion facets are arranged perpendicular to the table to display a contrasting secondary hue.
- Trilliant or triangular cuts provide three optical viewing directions to highlight trichroism.
- Mixed cuts with step facets optimize light reflection at different angles.
- Some pear and marquise cuts align along pleochroic axes.
Additionally, the depth of cut influences color change visibility. Shallow stones tend to wash out the secondary hues from the pavilion. While overly deep stones darken the primary table color. Optimal cutting enhances the color change while maintaining sufficient transparency.
Enhancing Pleochroic Gem Value
The allure of color change contributes substantially to the value of pleochroic gems like alexandrite and tanzanite. Both the number of observable colors and the distinctness of color shifts are key pricing factors. Some guidelines for assessing pleochroism include:
- Strength – Visible from all angles? Under all light types?
- Number of colors – Dichroic or trichroic?
- Color range – Greater change merits higher value.
- Cut quality – Does it highlight the color change?
- Clarity – Transparent with few inclusions?
- Carat weight – Larger gems command more value.
- Origin – Natural gems have higher prices than synthetic.
With the same other quality factors, a vividly pleochroic gem will be worth more than a non-pleochroic equivalent. However, pricing remains heavily influenced by supply levels for a particular gem variety and locale.
Conclusion
The special optical properties of pleochroic gems enable them to show remarkable color changes. While the effect occurs to some extent in many gem types, it is most prized in rare varieties like alexandrite and tanzanite. With proper orientation, cutting, and lighting, color-change gems can display their full pleochroic potential. The unique visual effect gives additional intrigue and value to these phenomenal gemstones.