Gemmology students known it for its educational potential to display a very important property in gem testing: pleochroism. Cut andalusites offer an eye-catching examples, bending beauty with education!
Andalusite is an aluminium silicate (sharing the same composition with kyanite and sillimanite) that occurs typically in reddish-brown to greenish colours namely in Brazil , Sri Lanka, Madagascar, Myanmar or USA.
Pleochroism, technically, is the selective absorption of visible light according to the crystallographic direction of certain coloured crystals (that are optically anisotropic materials belonging to the triclinic, monoclinic, orthorhombic, tetragonal and hexagonal crystallographic systems… still with me folks!?).
This basically mean that the material will appear to have different colours when inspected along specific different directions.
Andalusite has a strong pleochroism that is so distinct that it is readily visible even without instruments or special filters (polarising filters) in a cut stone and it is quite diagnostic for this gem material.
Knowing this property is relevant for cutters to properly plan the cut and chose the right orientation of the design in order to obtain optimal visual performance.
Andalusite also has a trapiche-like variety, known as chiastolite, with a cross-like dark pattern in cross section that occurs, for example, in Spain (Santiago de Compostela, Asturias), being also known in the trade as piedra-cruz, lapis crucifer or Compostela cross stone.
In the photo, a magnificent 10.41 ct andalusite set in a white gold and diamond ring by Kat Florence. Note how the reddish colour co-exists with the greens in this direction of observation (face up position) as seen in the pavilion facets © Kat Florence Jewels ; chiastolite (andalusite) from Sterling, Massachussetts, USA © Weinrich Minerals ; andalusite crystal from Mimoso do Sul, Espirito Santo, Brazil © The Arkenstone, www.iRocks.com
Guest articles by Rui Galopim de Carvalho:
https://gem-spectrum.com/category/rui-galopim-de-carvalho
About the author:
Rui Galopim de Carvalho is a renowned independent Gem Education Consultant, author, luxury industry training expert and popular international lecturer on gemmology and on the history of gem materials. During his 30 year experience as a gemmologist, Rui has studied important collections in both public and private museums, notably the Portuguese Crown Jewels.
Founder of the popular Home Gemmology webinar series launched during the COVID-19 lockdown, Rui has a long history as a gemmology communicator, notably on social media where he is a popular figure.
Rui Galopim de Carvalho can be reached through his website:
