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...

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