Pleochroism is an

optical phenomenon Optical phenomena are any observable events that result from the interaction of light and matter. All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the Sun or Moon with ...

in which a substance has different colors when observed at different angles, especially with polarized light.

Etymology

The roots of the word are from Greek (). It was first made compound in the German term ''Pleochroismus'' by mineralogist Wilhelm Haidinger in 1854, in the journal ''Annalen der Physik und Chemie''. Its first known English usage is by geologist James Dana in 1854.

Background

Anisotropic Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...

crystals will have optical properties that vary with the direction of light. The direction of the electric field determines the polarization of light, and crystals will respond in different ways if this angle is changed. These kinds of crystals have one or two optical axes. If absorption of light varies with the angle relative to the optical axis in a crystal then pleochroism results. Anisotropic crystals have double refraction of

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

where light of different polarizations is bent different amounts by the

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

, and therefore follows different paths through the crystal. The components of a divided light beam follow different paths within the mineral and travel at different speeds. When the mineral is observed at some angle, light following some combination of paths and polarizations will be present, each of which will have had light of different colors absorbed. At another angle, the light passing through the crystal will be composed of another combination of light paths and polarizations, each with their own color. The light passing through the mineral will therefore have different colors when it is viewed from different angles, making the stone seem to be of different colors.

Tetragonal In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the Cube (geometry), cube becomes a rectangular Pri ...

trigonal In crystallography, the hexagonal crystal family is one of the six crystal family, crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the tr ...

, and

hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A regular hexagon is d ...

minerals can only show two colors and are called

dichroic In optics, a dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths (colours) (not to be confused with dispersion), or one in which light rays having different polarizations are ab ...

Orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic Lattice (group), lattices result from stretching a cubic crystal system, cubic lattice along two of its orthogonal pairs by two different factors, res ...

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

, and

triclinic class=skin-invert-image, 180px, Triclinic (a ≠ b ≠ c ≠ a and α, β, γ, 90° pairwise different) In crystallography, the triclinic (or anorthic) crystal system is one of the seven crystal systems. A crystal system is described by three b ...

crystals can show three and are trichroic. For example, hypersthene, which has two optical axes, can have a red, yellow, or blue appearance when oriented in three different ways in three-dimensional space. Isometric minerals cannot exhibit pleochroism.

Tourmaline Tourmaline ( ) is a crystalline silicate mineral, silicate mineral group in which boron is chemical compound, compounded with chemical element, elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. This gemstone comes in a ...

is notable for exhibiting strong pleochroism. Gems are sometimes cut and set either to display pleochroism or to hide it, depending on the colors and their attractiveness. The pleochroic colors are at their maximum when light is polarized parallel with a principal optical vector. The axes are designated X, Y, and Z for direction, and alpha, beta, and gamma in magnitude of the refractive index. These axes can be determined from the appearance of a crystal in a conoscopic interference pattern. Where there are two optical axes, the acute bisectrix of the axes gives Z for positive minerals and X for negative minerals and the obtuse bisectrix gives the alternative axis (X or Z). Perpendicular to these is the Y axis. The color is measured with the polarization parallel to each direction. An absorption formula records the amount of absorption parallel to each axis in the form of X < Y < Z with the left most having the least absorption and the rightmost the most.

In mineralogy and gemology

Pleochroism is an extremely useful tool in

mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical mineralogy, optical) properties of minerals and mineralized artifact (archaeology), artifacts. Specific s ...

and

gemology Gemology or gemmology is the science dealing with natural and artificial gemstone materials. It is a specific interdisciplinary branch of mineralogy. Some jewellery, jewelers (and many non-jewelers) are academically trained gemologists and are qua ...

for

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

and gem identification, since the number of colors visible from different angles can identify the possible crystalline structure of a gemstone or mineral and therefore help to classify it. Minerals that are otherwise very similar often have very different pleochroic color schemes. In such cases, a

thin section In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron ...

of the mineral is used and examined under polarized transmitted light with a petrographic microscope. Another device using this property to identify minerals is the

dichroscope A dichroscope is a pocket instrument used in the field of gemology, and can be used to test transparent gemstones (crystals). Experienced gemologists, observing the pleochroism of some gems, can successfully detect gemstones from other artificial ...

.What is gemstone pleochroism?
International Gem Society, retrieved 28-Feb-2015

List of pleochroic minerals

Purple and violet

Amethyst Amethyst is a Violet (color), violet variety of quartz. The name comes from the Koine Greek from - , "not" and (Ancient Greek) / (Modern Greek), "intoxicate", a reference to the belief that the stone protected its owner from Alcohol into ...

(very low): different shades of purple * Andalusite (strong): green-brown / dark red / purple *

Beryl Beryl ( ) is a mineral composed of beryllium aluminium Silicate minerals#Cyclosilicates, silicate with the chemical formula Be3Al2(SiO3)6. Well-known varieties of beryl include emerald and Aquamarine (gem), aquamarine. Naturally occurring Hex ...

(medium): purple / colorless *

Corundum Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium, and chromium. It is a rock (geology), rock-forming mineral. It is a naturally transparency and translucency, transparent material, but ...

(high): purple / orange * Hypersthene (strong): purple / orange * Spodumene (

Kunzite Spodumene is a pyroxene mineral consisting of lithium aluminium Silicate minerals#Inosilicates, inosilicate, lithium, Lialuminum, Al(silicon, Sioxygen, O3)2, and is a commercially important source of lithium. It occurs as colorless to yellowish, ...

) (strong): purple / clear / pink *

(strong): pale purple / purple * Putnisite: pale purple / bluish grey

Blue

* Aquamarine (medium): clear / light blue, or light blue / dark blue * Alexandrite (strong): dark red-purple / orange / green *

Apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...

(strong): blue-yellow / blue-colorless * Benitoite (strong): colorless / dark blue *

Cordierite Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present, and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: to . A high-tempera ...

(aka Iolite) (orthorhombic; very strong): pale yellow / violet / pale blue *

(strong): dark violet-blue / light blue-green * Tanzanite See Zoisite *

Topaz Topaz is a silicate mineral made of aluminium, aluminum and fluorine with the chemical formula aluminium, Alsilicon, Sioxygen, O(fluorine, F, hydroxide, OH). It is used as a gemstone in jewelry and other adornments. Common topaz in its natural ...

(very low): colorless / pale blue / pink *

(strong): dark blue / light blue * Zoisite (strong): blue / red-purple / yellow-green *

Zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...

(strong): blue / clear / gray

Green

* Alexandrite (strong): dark red / orange / green * Andalusite (strong): brown-green / dark red *

(strong): green / yellow-green *

Emerald Emerald is a gemstone and a variety of the mineral beryl (Be3Al2(SiO3)6) colored green by trace amounts of chromium or sometimes vanadium.Hurlbut, Cornelius S. Jr., and Kammerling, Robert C. (1991). ''Gemology'', John Wiley & Sons, New York ...

(strong): green / blue-green * Peridot (low): yellow-green / green / colorless * Titanite (medium): brown-green / blue-green *

(strong): blue-green / brown-green / yellow-green *

(low): greenish brown / green * Kornerupine (strong): green / pale yellowish-brown / reddish-brown * Hiddenite (strong): blue-green / emerald-green / yellow-green

Yellow

* Citrine (very weak): different shades of pale yellow *

Chrysoberyl The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula Be Al2 O4. The name chrysoberyl is derived from the Greek words χρυσός ''chrysos'' and βήρυλλος ''beryllos'', meaning "a gold-white spar". Despit ...

(very weak): red-yellow / yellow-green / green *

(weak): yellow / pale yellow * Danburite (weak): very pale yellow / pale yellow *

Kasolite Kasolite is an uncommon lead uranyl silicate monohydrate mineral. It is an IMA approved mineral, that had been a valid species before the foundation of the association, that had been first described and published in 1921 by Schoep. It is a grand ...

(weak): pale yellow / grey *

Orthoclase Orthoclase, or orthoclase feldspar ( endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture", because its two cleavage planes are at right angles ...

(weak): different shades of pale yellow * Phenacite (medium): colorless / yellow-orange * Spodumene (medium): different shades of pale yellow *

(medium): tan / yellow / yellow-orange *

(medium): pale yellow / dark yellow *

(weak): tan / yellow *

Hornblende Hornblende is a complex silicate minerals#Inosilicates, inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common ...

(strong): light green / dark green / yellow / brown * Segnitite (weak): pale to medium yellow

Brown and orange

(strong): yellow-brown / orange *

(medium): brown-yellow / dull brown-yellow *

(very low): dark brown / light brown *

(very weak): brown-red / brown-yellow *

Biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more al ...

(medium): brown

Red and pink

* Alexandrite (strong): dark red / orange / green * Andalusite (strong): dark red / brown-red *

(strong): violet-red / orange-red * Morganite (medium): light red / red-violet *

(strong): dark red / light red *

(medium): purple / red-brown

References

{{reflist *“Pleochroism.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/pleochroism. Accessed 1 Jan. 2024. * “Pleochroism, N.” Oxford English Dictionary, Oxford UP, July 2023, https://doi.org/10.1093/OED/5173776922. Accessed 1 Jan 2024. Mineralogy Optical mineralogy Microscopy Gemology Lists of colors