Gem News International Gems & Gemology, Winter 2013, Vol. 49, No. 4

Bright Yellow Diamonds from Sierra Leone

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Figure 1. These two bright yellow diamonds (1.03 and 1.02 ct) are from alluvial deposits in the Zimmi mining area in southern Sierra Leone. In the rectangular stone, several black graphitized feather inclusions are visible. Photo by Robert Weldon.
Diamonds referred to in the trade as “Canary yellow” are highly valued for their saturated yellow color, caused by isolated nitrogen impurities. When faceted, they can exhibit colors ranging from Fancy Intense to Fancy Vivid yellow in GIA’s colored diamond grading system. It is unusual to see a group of these rare colored diamonds from a known mining locality (figure 1).

Recently, GIA’s Carlsbad laboratory had the opportunity to examine eight faceted and five rough diamonds reportedly found in alluvial deposits in the Zimmi mining area in the Pujehun district of southern Sierra Leone. They were loaned to us for study by Israel Eliezri (Coldiam Ltd., Ramat Gan, Israel). According to Mr. Eliezri, bright yellow rough diamonds are recovered on a recurring basis from the Zimmi region.

The diamonds we examined showed highly saturated yellow colors and weighed between ~0.10 ct and just over 1.00 ct. Rough diamonds recovered from this locality are reported to occur in sizes up to about 4 ct. Under magnification, they often contain what appear to be crystalline sulfide mineral inclusions, with graphitized feathers extending outward (figure 2). Infrared spectroscopy analysis indicated that the samples ranged from pure type Ib to variable proportions of type Ib and IaA. Total nitrogen impurity concentrations varied from 8 ppm (all as type Ib) to ~340 ppm (~35 ppm type Ib, ~305 ppm type IaA). Hydrogen impurities were also detected in the samples with high total nitrogen content. Visible spectroscopy showed only gradually increasing absorption toward the violet end of the spectrum due to the isolated nitrogen.

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Figure 2. This diamond contains what is believed to be a metallic sulfide mineral inclusion, surrounded by a black, graphitized feather. Photomicrograph by Nathan Renfro; horizontal field of view is 1.08 mm.
Many yellow natural diamonds with isolated nitrogen occur as irregularly shaped rough crystals, often with inward faces in the form of re-entrant cubes. The rough diamonds from the Zimmi mine, however, showed smooth and rounded dodecahedral shapes. This rounded shape makes them much more suitable for cutting with minimal weight loss.

Type Ib diamonds are uncommon because with time and heat, isolated nitrogen impurities in the lattice tend to aggregate, forming more-complex defects that are characteristic of type IaA and IaB diamonds. The youngest dated diamonds are about a billion years old, and the youngest emplacement ages of diamondiferous kimberlites are less than 80 million years old. This suggests that nearly all diamonds spend most of their existence at great depths in the lower crust and upper mantle. It is unclear exactly how the isolated nitrogen impurities in type Ib diamonds avoided the aggregation process. This mystery makes the discovery of many “Canary yellow” diamonds from a single mine in Sierra Leone even more interesting, offering a potential tool for research into the formation and survival of type Ib diamonds in nature.

James Shigley is a distinguished research fellow, and Christopher M. Breeding is a research scientist, at GIA's laboratory in Carlsbad, California.