T172 Gemological Research in the 21st Century—Characterization, Exploration, and Geological Significance of Diamonds and other Gem Minerals (Posters)

Monday, 23 October 2017
9:00 AM - 6:30 PM
Halls 4EF

THE 812-CARAT PURE TYPE IAB CONSTELLATION DIAMOND FROM KAROWE – PART OF AN EVEN LARGER ROUGH?

Paper number: T172-9
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Lucara Diamond extracted an 812 ct colorless diamond from the AK6 pipe at Karowe mine in November 2015. Named “The Constellation,” it is the 6th largest gem-quality rough discovered, and attained the highest price for a rough, selling in May 2016 to Nemesis International DMCC for $63.1 million ($77,649 per carat). Interestingly, several large diamonds were recovered at Karowe within the same week, including the 2nd largest gem-quality rough, the 1,109 ct Lesedi La Rona, as well as other colorless diamonds weighing 374, 296, and 183 ct.

The Constellation and the 296 and 183 ct diamonds were studied using a combination of gemological and spectroscopic techniques. Diamonds are classified according to their nitrogen content measured by FTIR spectroscopy: Type I diamonds contain nitrogen in either isolated (Ib) or aggregated (IaAB) forms, while Type II diamonds do not contain detectable nitrogen concentrations (IIa), but may contain boron (IIb). All 3 were pure Type IaB, containing 20 (4) ppm B-centers. PL spectroscopy detected emission from H4, H3 and GR1 centers, with similar peak characteristics.

The external morphologies of the stones showed primary octahedral, resorbed, and fractured faces, with both The Constellation and 296 ct diamonds featuring a fracture containing metallic inclusions and secondary iron-oxide staining. The internal growth structures were investigated using UV fluorescence imaging, and each sample contained at least 2 distinct growth zones.

These data suggest that these 3 specimens originate from the same rough, which may have broken either during kimberlite eruption or mining processes. With a combined weight of 1,291 ct, the composite would have surpassed the Lesedi La Rona. These diamonds are notable for being the largest pure Type IaB samples reported, whereas large colorless diamonds are often highlighted as Type IIa. We analyzed 5,060 colorless to faintly colored (D–Z) faceted diamonds weighing between 10.0 and 228.3 ct. As expected, Type IaAB (73.8%) diamonds were most common. However, only 1.2% of diamonds were pure Type IaB, significantly rarer than Type IIa diamonds (24.6%).

This report will demonstrate how multi-technique analyses of point defects and their distributions can link multiple rough diamonds comprising a single diamond of remarkable historic and scientific significance.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-305275

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