NATURALLY IRRADIATED BLACK DIAMONDS FROM MARANGE (ZIMBABWE)

Black colour in diamond can have both natural and treated origins. Natural black diamonds can be coloured by dark inclusions, and a rare natural diamond coloured by brown radiation stains was previously examined by GIA’s Carlsbad Lab¹. Treated black diamonds are often heavily fractured diamonds that are treated at low-pressure and high-temperature (LPHT) to graphitise the fractures and turn them black. Artificial irradiation can also produce dark colours that appear black, and in some cases black colour can be introduced through coating. The challenge for gem labs is to identify these treated black diamonds from black diamonds that occur naturally.

Here we investigate the colour origin of 40 natural brown-black Marange diamonds. Visual observations show that these diamonds are coloured by a combination of graphite micro-inclusions (associated with methane), graphite needles and brown radiation stains occurring along fractures. GR1 (V⁰), typically formed during natural and artificial irradiation, is observed in the visible spectra of 43% of the diamonds, however the concentration is too low to influence the body colour. Aside from radiation staining, irradiation-damaged diamond appears non-fluorescent in DiamondView images. Natural irradiation is related to the diamonds' billion year residence in a conglomerate that is known to contain radioactive minerals. Brown radiation stains form due to heating of the diamonds during later regional metamorphism, which also facilitated H3 (NVN⁰) and NiN formation.

A sub-set of diamonds contain ’clouds’ of graphite+methane that give the diamonds a dark appearance and lowers their value. Heat treatment of these graphitic diamonds can introduce gem-quality treated black diamonds into the market. Naturally, these graphite micro-inclusions are around 1 μm after heating above 1200 °C they become larger. Above 1700 °C, the grain size increases to 11-16 μm resulting in an opaque black appearance, and disappearance of any associated methane². Accordingly, visual observations of graphite grain size, along with FTIR and Raman spectroscopy to detect methane, are required to distinguish natural from heat-treated Marange graphitic diamonds.

References

1. Ardon, T., 2013. Gems and Gemology, 49(4), Lab Notes.

2. Eaton-Magaña, S., et al., 2017. Diamond and Related Materials, 71, 20-29