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Posted by David
Early last month, Shore Gold (SGF) announced that a high proportion (26%) of diamonds >2.7 c retrieved from the underground bulk sample at its 100% owned Star kimberlite in Saskatchewan are type IIa. This is a category of diamond that is typical of many “large special” diamonds >10.8 carats in size.
In terms of impurities in their crystal structure, diamond can substitute nitrogen (N), boron (B), and/or hydrogen (H) for carbon. Nitrogen is the most abundant and well-studied impurity and can range from concentrations of 0 to >10,000 ppm (~1%). Diamonds with significant nitrogen (>10 ppm) are termed Type I and those without are Type II. N-bearing diamonds are further categorized into those where the substituting N is organized as single atoms (Type Ib) or as aggregates of more than one atom (Type Ia). These aggregates are classified into paired N atoms (Type IaA) or quartets (Type IaB), or a mix of both (Type IaAB).
Diamonds that are relatively free of N are Type II. Those with no N and some B are Type IIb. Type IIa diamonds are more common and have no N or B. Type Ib and IIb diamonds are relatively rare. Type Ia diamonds are the most common.
How Diamond Types Are Determined
How impurities such as nitrogen are arranged in a diamond can be determined in a non-destructive manner using Fourier-transform infra-Red (FTIR) spectroscopy. Simply, light of a lower energy than visible light (infra-red) is shone through the diamond. By measuring the exact amount of light of a given energy that comes out the other side of the diamond (i.e. how much light is absorbed), it is possible to learn things about the diamond’s molecular structure. For example, how much nitrogen is in the diamond, and if it is in atomic pairs, or quartets. Fourier-transform is a mathematical and instrumental technique applied to infrared spectrometry to speed up analyses.
Issues With The Report’s Interpretation
In their news release, SGF refers to the Letšeng-la-Terae (Letšeng) mine in Lesotho (operated by Gem Diamonds, LSE-GEMD). This mine is considered quite unique as its low grade – <0.04 c/t, but has diamonds impressive quality and size. Average diamond value for this mine is >US$2000/c. This means a revenue of ~$80/t (2008 values).
However, the report’s suggestion that Type IIa equates to higher value stones cannot be considered absolute fact. This is because the mine they are comparing their diamonds to – Letšeng, is an anomaly in terms of its diamond population. While it is possible that with further valuation of parcels for SGF pipes a higher valuation could be realized, the current one is only about 10% (~$225/c) of Letšeng’s.
The diamonds shown by SGF in the full report (see above image for an example)- while large, are typically yellow-brown and some appear to contain large inclusions (internal cracks or non-diamond minerals). The report goes on to compare Letšeng and Star diamonds in terms of size class and % Type IIa. While Letšeng does show a marked increase in % Type IIa with increasing size, Star shows only a marginal increase, if at all.
The FTIR report commissioned by SGF also makes an error when referring to the trend of increasing percentage of Type IIa diamonds with increasing carat size for Star as comparable to that of Letšeng. The trends for each pipe are in fact rather different. Letšeng shows a significant increase of the proportion of Type IIa diamonds with size, whereas Star shows only a marginal increase (see plot below).
The SGF report states that the above figure “shows explicity that the abundance of Type II diamonds increases with increasing diamond size.” This statement is misleading as it is really only true for Letšeng diamonds. The academic study on Letšeng diamonds that SGF references for this report was based on less than 500 diamond samples (large stones of value being hard to obtain even for non-destructive studies). This relatively small number means that care must be taken when applying this study on a small number of diamonds from one kimberlite to the entire potential production of another. Granted, not that many large diamonds have been made available for such studies, but such over-reaching statements should not be made.
While the results of the report are interesting, and parallels can be made with the academic paper on Letšeng, there does not appear to be much evidence at this point for increased financial prospects of the Star project in terms of diamond type. Star still has one tenth the average diamond valuation of Letšeng without having close to ten times the grade. Though this does not in any way forestall a diamond mine in Saskatchewan, far better numbers have to come out of the Fort à la Corne area kimberlites for it to approach the level of Letšeng.
Disclaimer: The author does not hold shares of any company mentioned in this article. Relevant comments are welcome and encouraged. Spam comments will be deleted. This article is based on the opinions and experience of the author. Please conduct due diligence when investing. ©KIM Report 2010 www.kimreport.com