Yellow diamonds, some with colourless cores like the one pictured here, are of interest to the gemstone industry.
A new study is providing critical insight into how rare, high-value yellow diamonds formed in the Canadian north. The research, conducted by University of Alberta scientists, examined the chemical makeup of yellow diamonds—yielding a better understanding of how they form, to help guide the search for the elusive stones.
“Without this research, we wouldn’t know that two separate formation events occured, and that the second, more recent event is responsible for the yellow colour,” explained Thomas Stachel, professor in the University of Alberta’s Department of Earth and Atmospheric Sciences. “The more we know about the origin of these potentially high-value diamonds, the better results for diamond exploration and value creation in Northern Canada.”
The study, led by graduate student Mei Yan Lai, examined the unique stones recovered from Chidliak and Ekati Mine in Northern Canada.
“We wanted to understand the colour origin of yellow diamonds from these two deposits,” said Lai, a PhD student who completed this research as part of her master’s studies in the Faculty of Science Diamond Exploration Research Training School (DERTS) under the supervision of Stachel. “Canadian yellow diamonds have never been studied spectroscopically in detail. Our results suggest that the cause is the preservation of unstable single nitrogen atoms preserved inside the diamonds”
Following intensive analysis, the research team determined that some yellow diamonds contain colourless cores, meaning that the yellow outer layers crystallized on top of clearer centres. Lai determined that the yellow diamonds crystallized no more than 30,000 years before the kimberlite eruptions that brought them up to Earth’s surface.
“Our analysis shows that the colourless cores in these yellow diamonds are about one billion years older,” added Lai. “In fact, the carbon isotope compositions and nitrogen concentrations of the colourless cores and yellow outer layers are significantly different, suggesting that they formed in at least two distinct events and involved different diamond forming fluids.”
Diamond in the rough
The discovery of a potential new source of yellow diamonds in Canada’s north is economically significant, as the previous principal source of high-quality yellow diamonds, the Ellendale Mine in Western Australia, was recently shut down. In addition, the discovery of colourless cores in some of the yellow diamonds may also be of interest to the jewelry trade. “Occasionally rough yellow diamonds lose their vibrant yellow colour after being cut and polished—probably because this kind of diamond has a thin layer of yellow overgrowth on top of the geologically older colourless core,” said Lai
This project is a collaboration with Dominion Diamond Mines and Peregrine Diamonds Ltd. Part of the analyses were done at the Gemological Institute of America (GIA). This research is supported by a bursary through DERTS, funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) Collaborative Research and Training Experience (CREATE) program.
The paper, “Yellow diamonds with colourless cores – evidence for episodic diamond growth beneath Chidliak and the Ekati Mine, Canada,” was published in Mineralogy and Petrology (doi: 10.1007/s00710-020-00693-0).