Chatoyancy, or cat’s-eye effect, is an optical phenomenon consisting of a line or bright band of light hovering above the surface of a curved, polished domed surface, like in a cabochon for example, caused by multiple parallel structures or inclusions (e.g. needle-like inclusions, hollow tubes).
This sheen effect is seen perpendicular to the direction of those parallel structures and reveals its best under a bright single light source like direct sunlight or a penlight.
The orientation of the base of the cabochon must be parallel to the direction of the inclusions to obtain a centred effect.
The cat’s-eye effect is commonly seen in many gemstones, including tourmaline, sillimanite, kornerupine, moonstone, quartz and opal, just to mention a few, and is highly valued in the yellow, brownish orange to grey chrysoberyl, sometimes known in the trade as cymophane, from Sri Lanka (Elahera) and Brazil.
Especially the honey-coloured stones with a well-centred and defined bluish-white chatoyant effect.
The rare colour-changing variety of chrysoberyl, alexandrite, may also occur with cat’s-eye effect and is highly sought after by connaisseurs.
In the photos, cat’s-eye chrysoberyl ring © Lang Antique and Estate Jewelry and a remarkable 9.57 ct example from Sri Lanka at the Natural History Museum of Los Angeles County, photo © Robert Weldon/GIA
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:
