Abstract
For this study, the properties of a large sample of various types of brown diamonds with a deformation-related (referred to as “DR” in this work) color were studied to properly characterize and classify such diamonds, and to compare them to pink to purple to red diamonds. The acquisition of low temperature NIR spectra for a large range of brown diamonds and photoexcitation studies combined with various treatment experiments have opened new windows into certain defect characteristics of brown diamonds, such as the amber centers and naturally occurring H1b and H1c centers. It was determined that the amber centers (referred to as “AC” in this work) exhibit rather variable behaviors to annealing and photoexcitation; the annealing temperature of these defects were determined to range from 1150 to >1850 °C and it was found that the 4063 cm−1 AC was the precursor defect of many other ACs. It is suggested that the amber centers in diamonds that contain at least some C centers are essentially identical to the ones seen in diamonds without C centers, but that they likely have a negative charge. The study of the naturally occurring H1b and H1c link them to the amber centers, specifically to the one at 4063 cm−1. Annealing experiments have shown that the H1b and H1c defects and the 4063 cm−1 AC were in line with each other. The obvious links between these defects points towards our suggestion that the H1b and H1c defects are standalone defects that consist of multiple vacancies and nitrogen and that they are—in the case of brown diamonds—a side product of the AC formation. A new classification of DR brown diamonds was elaborated that separates the diamonds in six different classes, depending on type and AC. This classification had been completed recently with the classification of brown diamonds with a non-deformation-related color (referred to as “NDR”), giving a total of 11 classes of brown diamonds.
Highlights
Brown diamonds (Figure 1) make up a considerable percentage of the diamonds produced worldwide, from certain deposits like the Argyle mine in Australia
The brown color in most diamonds is associated with post-growth plastic deformation and the defect responsible for this color of such plastically deformed diamonds was identified as large spherical aggregates consisting of 40 to 60 vacancies, so-called vacancy clusters [2,3,4,5]
When annealing a type Ib DR brown diamond with distinct ACs to the higher temperatures of 1850 ◦ C we found the same results as described above concerning the ACs at 4063 cm−1, 4167 cm−1, 4093 cm−1 and 4111 cm−1 ; no consistent annealing behavior for the 3462 cm−1 AC could be determined, since it appears to behave randomly, intensifying in one sample while in a another one it decreased
Summary
Brown diamonds (Figure 1) make up a considerable percentage of the diamonds produced worldwide, from certain deposits like the Argyle mine in Australia. They represent a large group with variable properties and were—in combination with this work—classified recently [1]. The brown color often exhibits modifying hues, such as yellow, orange, green, and pink. The brown color in most diamonds is associated with post-growth plastic deformation and the defect responsible for this color of such plastically deformed diamonds was identified as large spherical aggregates consisting of 40 to 60 vacancies, so-called vacancy clusters [2,3,4,5]. Not all brown diamonds are plastically deformed, though, as there are CO2 and “Pseudo CO2 ” diamonds [6] as well as hydrogen-rich diamonds [7] that owe their brown color to other defects
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