Zircon may be one of the most under-appreciated coloured gemstones and please do not confuse it with “cubic zirconia”, or CZ (an artificial product with no stable natural counterpart) from the mid-1970s.
Zircon occurs in many rock types and is quite resistant to geological events, so resistant that the oldest terrestrial mineral on record is an Australian zircon with 4.374 Ga (Giga annum = billion years) of age, just a bit younger than our planet Earth itself (4.56 Ga). It has sub-adamantine lustre (meaning that its polished surfaces reflect light almost as intensely as in a diamond)) and high dispersion (0.039), close to diamond’s, and a very high birefringence (0.059) that causes the typical “doubling” than one often observed through the table facet.
As a gemstone it occurs in many colours, including yellow, brown, orange, red, green, colourless (the so-called “Matara diamonds”) and blue that is its most appreciated. These blues come especially from secondary deposits in the Ratanakiri Province in north-eastern Cambodia.
The colour results from the heat treatment of brown material at 900-1000ºC in reducing conditions for approximately one hour. The blue hue may fade to brown with exposure to long-wave ultraviolet light (365 nm) and returns back to blue under incandescent light after a few minutes, being hence an unusual tenebrescent stone interesting to play with.
In the photos: A blue zircon, emerald and single-cut diamond brooch by JAR © Christie’s ;
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:
