Introduction

Pleochroism is, to me, an underappreciated physical property of minerals. It is difficult to find information on what examples of pleochroism can be found on the market for collectors; many times you can only find information relevant in the context of thin-section optical mineralogy, or information that only applies to specimens from a specific locale will be given as general (for instance titanite will be said to have weak pleochroism even though some material has pleochroism on par with anything else in terms of strength). I plan to compile a list of minerals that can commonly be found and that exhibit pleochroism, as well as pictures or videos of as many as I can afford/find.

All photographs and videos are taken by me, copyright owned by me, of specimens in my collection.

A very brief, very basic explanation:

All of these crystals split light that is transmitted through them. The light gets split into two parts that are polarized differently (one wave goes "up and down" while the other goes "left to right"). The two "rays" vibrate in perpendicular directions and are also different colors! This can be visible with your bare eyes, but polaroid film (which is a linear polarizing filter, it only allows light through if it's vibrating in one of the two possible directions) enhances the effect by letting you isolate the polarized components.

It's essentially the same kind of effect as birefringence you're likely familiar with in calcite, but instead of the polarized rays experiencing different refractive indices along their distinct paths, the rays are selectively absorbed (and thus transmitted) differently and thus you get distinct transmission spectra.

The crystal must have axes that are asymmetric for it do have pleochroism, as the difference in transmission spectra is caused by differences in how light interacts with the material as it is transmitted along the axes. Light is selectively absorbed when it interacts with electrons, and light being transmitted through a crystal thus travels along these axes in the crystal's electronic structure.

This means that isometric minerals can not exhibit pleochroism, uniaxial minerals can display up to two pleochroic colors (dichroism) and biaxial minerals can exhibit up to three pleochroic colors (trichroism).

A disclaimer about 'dichroic' and 'dichromatic' glasses, synthetics, and the use of these terms generally

In the context of gemmology and optical mineralogy and geology more broadly, 'dichroism' is pleochroism with two apparent color axes and 'trichroism' is pleochroism with three apparent color axes. Uniaxial minerals can be at most dichroic (tourmaline), biaxial minerals can be up to trichroic (tanzanite).

In glass there are various metameric effects, scattering effects, interference effects, etc. that are referred to as 'dichroism' or 'dichromatism.' These are NOT dichroism in the sense of pleochroism. Pleochroism requires anisotropic crystal structure, glass is isotropic and amorphous and certainly not crystalline. Pleochroism results from broken periodic symmetries relative to isometric crystal symmetries but still requires clearly defined, unambiguous, anisotropic regularities.

Glass is said to be 'dichromatic' when it can show two different colors depending on lighting condition. For instance it may be red in transmitted light and green in reflected light, or one of many other schemes. This is effectively equivalent to opalescence, it only requires the presence of colloids. The glass may also be 'metameric' like color-change garnet, where a peaky absorption spectrum results in visually distinct colors depending on the color temperature and spectrum of the incident lighting conditions (or looks the same under different conditions under which typical materials don't look the same).

Thin-film interference effects, diffraction effects, etc. caused by coatings or colloids are often referred to as 'dichroism,' but again none of these have anything to do with pleochroism. The meaning of 'dichroism' and 'dichromatism' in the context of glasses and isometric materials generally often means nothing more substantive or rigorous than 'multi-colored.'

Alexandrite

Amethyst

SiO₂

Andalusite

Al₂(SiO₄)O

These two smaller pieces of andalusite were broken off of the larger piece seen in the plain-light video. From Brazil.

Axinite

Formula not found!

Cordierite

(Mg,Fe)₂Al₃(AlSi₅O₁₈)

Cordierite, also known as "iolite" particularly when it has a pronounced blue character, is one of the most commonly known and easily available examples of pleochroism. It is strong enough to be clearly apparent in all but the lightest material, although it can be so deeply saturated as to appear basically black without very strong backlighting. Its crystal structure is biaxial which means that cordierite can have up to three distinct transmission spectra along three axes.

The most common color axis to see in cordierite is its characteristic violet to violet-blue axis. While owing its hues to iron impurities as opposed to vanadium impurities, its blue and purple bear a striking resemblance to tanzanite.

In most material there will be a smoky gray to light yellow axis that is also easily discernible. While technically capable of trichroism, most material will appear predominately dichroic with two clear and distinct color axes, one a richly saturated violet to blue-violet, the second a gray, colorless, or vaguely yellow axis.
Cordierite with pronounced trichroism will typically have a predominately violet axis, a predominately blue axis, and the third axis will be colorless/gray/yellow.
In some material the yellow axis will be richly saturated which provides for striking contrast when viewed with a dichroscope, polaroid film, or a polarized light source.

More rare is cordierite with one or more orange-red to red axes. In some there is no pure violet to blue-violet axis, instead showing one axis with a mixture of violet and orange-red. More and more of this material can be found for sale online, but the prices are many times what you would expect for predominately violet stones.

Cordierite can also be found with aventurescent inclusions in the form of cabochons, often sold as "bloodshot iolite" or "iolite sunstone."

https://onlinelibrary.wiley.com/doi/full/10.1002/jcc.27288
https://www.gia.edu/gems-gemology/spring-2016-gemnews-red-cordierite-madagascar

Enstatite

While not all enstatite specimens will show pronounced pleochroism, some can be found with a very stark red to green axis only apparent with the use of polaroid film or a calcite dichroscope.

Specimens sold as enstatite are unlikely to truly be enstatite; enstatite is the magnesium end-member of the orthopyroxene solid solution series with ferrosilite as the iron end-member. Intermediate members that are Mg-rich are called hypersthene, intermediate members that are Fe-rich are called ferrosilite. Any real-world specimen is much more likely to be hypersthene or ferrosilite than it is to be enstatite.

I'll leave this entry as "enstatite" because that's what I was sold this as, and because that's what seems to be the most common term being used commercially. Given that pleochroism in orthopyroxene minerals requires iron, I don't think it's technically enstatite, and should more properly be labeled hypersthene or ferroan enstatite.

This can all be quite confusing, but these pages should clarify it all.

https://www2.tulane.edu/~sanelson/eens211/inosilicates.htm
https://www.alexstrekeisen.it/english/pluto/pyroxene.php

Orthopyroxene



https://www.cambridge.org/core/journals/mineralogical-magazine-and-journal-of-the-mineralogical-society/article/abs/origin-of-optical-pleochroism-in-orthopyroxenes/AE3A9527682D88DCEF97839EBD288A46

Epidote

(CaCa)(AlAlFe³⁺)O[Si₂O₇][SiO₄](OH)

This epidote is extremely dark green, appearing black typically, but with strong back-lighting its color and pleochroism are highly apparent. This specimen is alternatively bright, intensely saturated "apple green" and a rich, dark amber-orange visible in a narrow range of angles. Twinning is made apparent by the alternating vertical bands of orange and green, and the growth hillocks on the photographed face make for an extremely clear demonstration of how color in pleochroic minerals depends on the crystallographic or optical axes along which the light is transmitted as the vivid green shines through the darker orange color through the inclined faces.

Image taken with no polarizing filter or light source, the splitting and separation of the light is entirely from the epidote itself.


This image was taken at an intermediate angle resulting in the dark-amber orange apparent in associated photo (Photo ID: 1362565) shifting to yellow.



Fluorapatite

Kornerupine

Mg₃Al₆(Si,Al,B)₅O₂₁(OH)

Kyanite

Al₂(SiO₄)O
Oregon sunstone

https://www.gia.edu/gems-gemology/winter-1991-sunstone-oregon-johnston

Pyromorphite (weak pleochroism, may remove)

Smoky quartz

SiO₂

Tanzanite

(CaCa)(AlAlAl)O[Si₂O₇][SiO₄](OH)


https://digital.car.chula.ac.th/chulaetd/4727/


This tanzanite specimen was photographed with a split polaroid dichroscope positioned such that all three trichroic colors can be seen at one time. With no polaroid filters you'd see predominately purple and yellow with blue muddling each. The polaroid filter essentially filters out the blue on the top half while isolating and presenting the previously muddling blue on the bottom half.

Titanite

CaTi(SiO₄)O
The combination of very high birefringence and very strong pleochroism in this titanite presents each polarized ray as a spatially separated, distinctly colored image of the culet as seen through the table.

Tourmaline

AD₃G₆ (T₆O₁₈)(BO₃)₃X₃Z

Tourmaline is probably the mineral most widely known for its pleochroism. Being uniaxial, tourmaline minerals can show up to two distinct pleochroic colors. As with its color and chemical composition, the pleochroism of tourmaline is incredibly variable. It's most commonly seen to change saturation/lightness without a change of huge eg one axis will be bright saturated green and the other will be black, or one axis will be medium or light pink and the second axis will be colorless. While pleochroism is typically strong, it's not typically visually interesting compared to something like tanzanite or ferroan enstatite.

Some tourmaline, though, like this sample of Brazilian tourmaline (possibly rubellite, possibly Baixo claim) shows very impressive contrast with one axis showing a strongly saturated pinkish red and the second axis showing a yellowish color. Specimen is from Minas Gerais, Brazil.

The pink color is likely caused by Mn3+ and Mn3+/Mn2+ IVCT, the yellow color is likely caused by Mn2+ (https://www.researchgate.net/publication/292671283_Color_change_of_tourmaline_by_heat_treatment_and_electron_beam_irradiation_UV-Visible_EPR_and_Mid-IR_spectroscopic_analyses).



This video shows the tourmaline with no polaroid filter:



Zircon

*Have video or picture in decent quality, need to upload

Some helpful links:
https://geo.libretexts.org/Bookshelves/Geology (Multiple full textbooks on mineralogy and gemology)

https://www2.tulane.edu/~sanelson/eens211/index.html (A webpage for a university mineralogy course with tons of helpful explanations and figures)

https://www.gemstonemagnetism.com/ (Great breakdown on the causes of color of axinite and tourmaline along with a good breakdown of the relevant solid solution series and species)

https://nature.berkeley.edu/classes/eps2/wisc/pleo.html (good reference, but often limited and difficult to interpret without seeing real examples)