Pleochroism (from Greek πλέων, ''pléōn'', "more" and χρῶμα, ''khrôma'', "color") 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 wit ...

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

Background

Anisotropic 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 Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefring ...

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

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, macro ...

, 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 becomes a rectangular prism with a squar ...

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

, 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'' has ...

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 ...

. Orthorhombic,

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

, and

triclinic 180px, Triclinic (a ≠ b ≠ c and α ≠ β ≠ γ ) In crystallography, the triclinic (or anorthic) crystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal i ...

crystals can show three and are trichroic. For example,

hypersthene Hypersthene is a common rock-forming inosilicate mineral belonging to the group of orthorhombic pyroxenes. Its chemical formula is . It is found in igneous and some metamorphic rocks as well as in stony and iron meteorites. Many references have f ...

, 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 group in which boron is compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is a gemstone and can be found in a wide variety of colors. The te ...

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 and

gemology Gemology or gemmology is the science dealing with natural and artificial gemstone materials. It is a geoscience and a branch of mineralogy. Some jewelers (and many non-jewelers) are academically trained gemologists and are qualified to identif ...

for

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

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 (very low): different shades of purple *

Andalusite Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5. This mineral was called andalousite by Delamétehrie, who thought it came from Andalusia, Spain. It soon became clear that it was a locality error, and that the spe ...

(strong): green-brown / dark red / purple *

Beryl Beryl ( ) is a mineral composed of beryllium aluminium silicate with the chemical formula Be3Al2Si6O18. Well-known varieties of beryl include emerald and aquamarine. Naturally occurring, hexagonal crystals of beryl can be up to several ...

(medium): purple / colorless * Corundum (high): purple / orange *

Hypersthene Hypersthene is a common rock-forming inosilicate mineral belonging to the group of orthorhombic pyroxenes. Its chemical formula is . It is found in igneous and some metamorphic rocks as well as in stony and iron meteorites. Many references have f ...

(strong): purple / orange *

Spodumene Spodumene is a pyroxene mineral consisting of lithium aluminium inosilicate, Li Al( Si O3)2, and is a source of lithium. It occurs as colorless to yellowish, purplish, or lilac kunzite (see below), yellowish-green or emerald-green hiddenite, pr ...

(

Kunzite Spodumene is a pyroxene mineral consisting of lithium aluminium inosilicate, Li Al( Si O3)2, and is a source of lithium. It occurs as colorless to yellowish, purplish, or lilac kunzite (see below), yellowish-green or emerald-green hiddenite, pr ...

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

Benitoite Benitoite () is a rare blue barium titanium cyclosilicate, found in hydrothermally altered serpentinite. It forms in low temperature, high pressure environments typical of subduction zones at convergent plate boundaries. Benitoite fluoresces ...

(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-temperat ...

(aka

Iolite 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-temperat ...

) (orthorhombic; very strong): pale yellow / violet / pale blue * Corundum (strong): dark violet-blue / light blue-green *

Tanzanite Tanzanite is the blue and violet variety of the mineral zoisite (a calcium aluminium hydroxyl sorosilicate), caused by small amounts of vanadium. Tanzanite belongs to the epidote mineral group. Tanzanite is only found in Simanjiro District of Ma ...

See

Zoisite Zoisite, first known as saualpite, after its type locality, is a calcium aluminium hydroxy sorosilicate belonging to the epidote group of minerals. Its chemical formula is Ca2 Al3( Si O4)(Si2O7)O(O H). Zoisite occurs as prismatic, orthorho ...

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

(strong): dark blue / light blue *

(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 t ...

(strong): blue / clear / gray

Green

* Alexandrite (strong): dark red / orange / green *

(strong): brown-green / dark red * Corundum (strong): green / yellow-green * Emerald (strong): green / blue-green * Peridot (low): yellow-green / green / colorless *

Titanite Titanite, or sphene (from the Greek ''sphenos'' (σφηνώ), meaning wedge), is a calcium titanium nesosilicate mineral, Ca Ti Si O5. Trace impurities of iron and aluminium are typically present. Also commonly present are rare earth metals ...

(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 Hiddenite is a pale-to-emerald green variety of spodumene that is sometimes used as a gemstone. The first specimens of the hiddenite variety of spodumene were recovered about 1879 near the tiny settlement of White Plains, west of Stony Point, A ...

(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". Despite ...

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

Danburite Danburite is a calcium boron silicate mineral with a chemical formula of CaB2(SiO4)2. It has a Mohs hardness of 7 to 7.5 and a specific gravity of 3.0. The mineral has an orthorhombic crystal form. It is usually colourless, like quartz, but ca ...

(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 *

(medium): different shades of pale yellow * Topaz (medium): tan / yellow / yellow-orange *

(medium): pale yellow / dark yellow *

(weak): tan / yellow *

Hornblende Hornblende is a complex 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 in igneous and metamorphic rock ...

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

Brown and orange

* Corundum (strong): yellow-brown / orange * Topaz (medium): brown-yellow / dull brown-yellow *

(very low): dark brown / light brown *

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

Red and pink

* Alexandrite (strong): dark red / orange / green *

(strong): dark red / brown-red * Corundum (strong): violet-red / orange-red *

Morganite Morganite is an orange or pink gemstone and is also a variety of beryl. Morganite can be mined in countries like Brazil, Afghanistan, Mozambique, Namibia, the United States, and Madagascar. Morganite has grown in popularity since 2010. ''Brides ...

(medium): light red / red-violet *

(strong): dark red / light red *

(medium): purple / red-brown

References

{{reflist Mineralogy Optical mineralogy Microscopy Gemology