Lab Notes Gems & Gemology, Summer 2013, Vol. 49, No. 2

Very Large Rough Diamond

This 1,138 ct single-crystal natural diamond (62.51 × 47.61 × 45.56 mm) is the largest ever examined by GIA. Photo by Joshua Balduf.
Throughout history, the discovery of very large diamond rough has been a rare occurrence. Not only is it unlikely to survive the journey from deep in the earth to the surface, but modern extraction methods, including the use of crushers to break large rock, can reduce such specimens to smaller pieces.

Therefore, the recent examination of a 1,138 ct rough diamond (62.51 × 47.61 × 45.56 mm; pictured above) provided a unique opportunity to document the largest single-crystal natural diamond ever examined by GIA. While it is possible there have been larger unreported specimens, a search of public sources (e.g., I. Balfour, Famous Diamonds, 5th ed., Antique Collectors Club, 2008) suggests this could be the second-largest rough diamond in history—between the 3,106 ct Cullinan and the 995 ct Excelsior, both of gem quality.

Known to be from the Democratic Republic of the Congo, this stone displayed Raman spectroscopic features characteristic of diamond. The sheer size presented a challenge with IR absorption spectroscopy analysis, as it was too large for the regular sample chamber, beam condenser, or DRIFT units. Instead, we used an infrared microscope with reflective mode and obtained a high-quality absorption spectrum. The rough was identified as a natural type IaA diamond with very high nitrogen concentration. We also detected a weak hydrogen-related absorption peak at 3107 cm–1, a common feature in natural diamond. Also recorded were strong absorption bands at approximately 3300, 2920, 1700, and 1550 cm–1, attributed to micron and sub-micron inclusions. Photoluminescence spectra were recorded at liquid-nitrogen temperature with various laser excitations. The main emission features were broad bands at 700 and 787 nm. The crystal also displayed a very weak blue fluorescence under long- and short-wave ultraviolet radiation, another typical feature. 

The irregular morphology of the 1,138 ct rough was quite interesting, as diamond usually occurs in octahedral or dodecahedral forms. But such a morphology is less likely to be maintained as a large crystal forms. Numerous randomly shaped and distributed inclusions may have contributed to the irregular crystal growth. These inclusions also caused the gray color appearance. 

The specimen’s strong surface dissolution provided a visual document of its journey from deep in the earth. At the high temperatures in the earth’s interior, diamond can remain stable only under very high pressure. As the diamond was transported to the surface, the decrease in pressure would have dissolved the stone unless it made the journey rapidly. The surface of the 1,138 ct crystal speaks to the change that took place during its journey and represents the fine line between our experiencing this rare item and never having known it at all.