Further geological work will be conducted at diamond exploration and project development company Botswana Diamonds’ Thorny River project, in Limpopo, to further refine grade and revenue estimates and extend volume estimates. “We have made rapid progress at Thorny River with the generation of volume and grade estimates following intensive exploration field work. [However,] we recovered insufficient diamonds, particularly the bigger sizes, to produce a valuation which could be used in economic modelling.

“This will be addressed. The next work programme will include options to determine the economic viability of the resource,” chairperson John Teeling noted in a media release issued on Thursday.

Recent modelling work undertaken by consulting group Interlaced Consulting on a sample equivalent to 305.38 t of kimberlite and host rock, resulted in a total content grade range of 46 carats per hundred tons (cpht) to 74 cpht.

The valuation of 512 diamonds recovered from the sample, which was undertaken by international diamantaire Ray Ferraris, indicated the parcel average to be $89/ct in the +11-diamond sieve class, which compares favourably with the $77/ct achieved in the same diamond sieve class from a 2015 sampling campaign

Given that the recovered diamonds were insufficient to produce a modelled diamond valuation; the 2016 modelled diamond value of $259/ct will continue to apply. Meanwhile, gem-quality diamonds comprised about two-thirds of the value of the parcel of 512 diamonds. Less than 40% of the parcel illustrated medium or high fluorescence and less than 10% of the diamonds were chipped or broken. Yellow diamonds were also present and there was a small population of unusual colour diamonds.

However, the company noted that it had expected to recover larger diamonds and would now investigate why that was not the case.

Along with an investigation into the absence of larger diamonds and work to tighten the grade range, the processed concentrates produced by the dense media separation (DMS) plant at Thorny River will also be subject to further work.