The Gemological Institute of America (GIA) recently encountered an exceptionally unusual diamond that defied easy explanation. The 2.50-carat round brilliant, graded at the institute’s Carlsbad, California, laboratory, displayed star-shaped inclusions so distinctive that researchers struggled to pinpoint their origin.

According to research “several randomly distributed yellow zones consisting of stacks of clouds with four-sided star patterns near the girdle noticed in the diamond.” These clouds—clusters of micro-inclusions—each contained a central concentration of more intensely colored particles arranged in a cross-like formation. When viewed from an alternate angle, the phenomenon appeared as a row of bright-yellow overlapping triangles, prompting the GIA to launch an in-depth analysis.

Initial observations revealed blue fluorescence under long-wave UV light, while the clouded yellow patches displayed weak yellow fluorescence under deep-UV imaging. Testing also indicated the presence of hydrogen-related defects, typically associated with brownish or greenish color components. However, these did not explain the intense yellow coloration of the zones, which is more commonly linked to cape defects, H3 centers, isolated nitrogen (C-centers), or a 480-nanometer absorption band.

To probe further, GIA scientists used photoluminescence spectroscopy to compare the yellow zones with surrounding areas. The results showed no signs of H3-related features or absorption-band activity. The only notable finding was a nitrogen-vacancy center present exclusively within the vivid yellow areas, suggesting the coloration stemmed from C-centers confined to specific surface regions.

The lab initially considered whether the diamond might resemble a case reported in 2020, in which a near-colorless diamond developed a yellow overgrowth layer late in formation. However, the composition and behavior of the current stone’s micro-inclusion clouds did not align with that scenario.

GIA researchers concluded that the origin and formation of the starburst-like micro-inclusions remain unknown — a mystery that will require further investigation.