Editing Pleochroism (section)

== Background ==
[[Anisotropy|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.<ref name="Bloss">{{cite book|last=Bloss|first=F. Donald|title=An Introduction to the Methods of Optical Crystallography|year=1961|publisher=Holt, Rinehart and Winston|location=New York|pages=147–149}}</ref>

Anisotropic crystals have [[double refraction]] of [[light]] where light of different [[Polarization (waves)|polarizations]] is bent different amounts by the [[crystal]], 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]], [[trigonal]], and [[Hexagonal crystal system|hexagonal]] minerals can only show two colors and are called [[dichroism|dichroic]]. [[Orthorhombic]], [[monoclinic]], and [[triclinic]] 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.<ref name="Bloss2">{{cite book|last=Bloss|first=F. Donald|title=An Introduction to the Methods of Optical Crystallography|year=1961|publisher=Holt, Rinehart and Winston|location=New York|pages=212–213}}</ref> [[Cubic crystal system|Isometric]] minerals cannot exhibit pleochroism.<ref name=Webmineral/><ref>{{ cite web|url=http://www.galleries.com/minerals/property/pleochro.htm |title=The Pleochroic Minerals |work=galleries.com}}</ref> [[Tourmaline]] 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.<ref>{{cite book|last1=Rogers|first1=Austin F.|last2=Kerr|first2=Paul F.|title=Optical Mineralogy|url=https://archive.org/details/opticalmineralog00roge|url-access=registration|edition=2|year=1942|publisher=McGraw Hill Book Company|pages=[https://archive.org/details/opticalmineralog00roge/page/113 113–114]}}</ref>