Asterism, also known in the trade as the star effect, is an optical sheen effect that occurs when two or more bright bands cross each other to produce a star impression on domed polished surfaces, like in a cabochon, for example.
The phenomenon is caused by more than two crystallographically oriented sets of minute parallel features (e.g. needle-like inclusions, growth tubes) generating intersecting chatoyant effects, hence a star effect that, in most cases, may have 4 rays (e.g. diopside, garnet), 6 rays (e.g. ruby, sapphire, garnet, quartz) or 12 rays (e.g. quartz, garnet).
The effect can be observed in both transmitted light (diasterism), but more frequently in reflected light (epiasterism) under a strong light bulb or in direct sunlight. As a light reflection related effect, it is hardly seen in diffused light, suggesting a wise choice of illuminant options in both testing and display. The orientation of the base of the cabochon must be parallel to the direction of the effect-causing structures to obtain a centred star that is critical to its desirability in gemstones.
Asterism may also be artificially induced, namely in natural or synthetic corundum by diffusion treatment or heat treatment at high-temperature (typically between 1100-1600ºC to as high as 1950ºC) by causing the exsolution of titanium oxide into a needle-like crystallographically orientated solid titanium-bearing phase, usually at surface level.
Star stones are interesting examples where understanding crystallography, physics and optics is part of the savoir-faire in the manufacturing and product display process.
In the images, the famous Rosser Reeves ruby, a large 138.7 ct cabochon from Sri Lanka with a very well-defined star, donated by Mr and Mrs Rosser Reeves to the Smithsonian in 1965, and the Star of India, a 329.7 ct star sapphire from Myanmar, photos by Chip Clark © Smithsonian National Museum of Natural History
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:
