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While the first part of this study took a detailed look at the properties, defects and classification of brown diamonds with deformation-related (DR) brown color and compared them to pink to purple to red diamonds, this second part covers diamonds with non-deformation-related (referred to as NDR in this study) brown color, including diamonds with treatment-induced brown color and synthetic brown diamonds. It was found that the natural NDR brown diamonds include CO2 and Pseudo CO2 diamonds as well as certain hydrogen-rich diamonds. Based on these, the new classification of NDR brown diamonds has been elaborated, resulting in 5 different classes. The detailed defect study performed has shown and confirmed the complexity of the CO2 and Pseudo CO2 diamonds; the probable link between structurally bound oxygen and some of the spectroscopic features such as the 480 nm absorption band is apparent in these diamonds. One of the most interesting findings was made through the low temperature NIR spectroscopy of some usually hydrogen-rich diamonds, which has defined a defect of great interest, the 1330 nm center; we suggest that this defect, together with the many lines in the 970 to 1000 nm range—referred to as the 990 nm series in this study—are responsible for the complex UV-Vis-NIR spectra seen of these diamonds. The results indicate that both features are nickel-nitrogen-related defects, the 1330 nm defect without involvement of hydrogen and the 990 nm series likely with hydrogen involved. Another surprising result was that during various treatment experiments performed we created dark orangish brown color in originally pale yellow “cape” diamonds by HPHT treatment at 2500 °C. It is suggested that the creation of this brown hue is related to the destruction or transformation of the N3 center at such extreme conditions.

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.

[...]there were significant differences in their UV-Vis-NIR absorption spectra (Figure 32). See PDF.] Semi-quantitative chemical analyses of the Afghan samples by EDXRF confirmed the identification of the haüyne species within the sodalite group, with mean relative proportions of Na, Ca and K of 70%, 29% and 1%, respectively. [...]arsenic was systematically detected in the Afghan haüyne samples, but was not found in the German material that we analysed, and likewise has not been reported in haüyne in the literature. [...]recently, gem-quality haüyne has been scarce and extremely expensive, but new production from Afghanistan is increasing the availability of this attractive gem material, including polished samples weighing 4–5 ct and more.

Since the latter half of 2023, yellow-orange to deep orange hackmanite from Badakhshan, Afghanistan, has appeared on the gem market. Eight faceted samples were examined for this report that were light yellowish orange to strong orange, and exposure to UV radiation caused them to become pinkish orange to orangey red. The addition of a purple colour component due to UV exposure is consistent with the photochromism typically observed in hackmanite. An absorption band at 480 nm is responsible for the orange hue, but it does not correspond to any other known colour centre in the sodalite structure. The presence of unaltered two- and three-phase fluid inclusions suggests that the samples had not been heated. Repeated exposure to UV radiation eventually causes the stable colour to fade (to light orangey yellow) and also diminishes the photochromic behavior, so we conclude that the 480 nm band is most likely due to irradiation, which could be natural, artificial or both. Irradiation experiments performed on faded samples prove that the unstable deep orange colour, as well as the photochromic behaviour, can be temporarily restored by exposure to 150 kGy of gamma-ray radiation.

The UV-Vis-NIR absorption spectrum displayed an absorption continuum that increased towards the UV region, as well as a relatively broad band with an apparent maximum at approximately 460 nm and a superimposed peak at 450 nm (Figure 47, orange trace). Semi-quantitative EDXRF analysis showed an Fe content of approximately 120 ppmw, which is relatively low in comparison to other sapphires of similar colour, but which are coloured only by Fe3+ (i.e. Fe3+−Fe3+ pairs). The presence of uranothorianite inclusions and the accompanying long-term exposure of the sapphire to traces of ionising radiation may provide an explanation for the photochromic property.

Minerals of the sodalite group (e.g. sodalite, haüyne, lazurite, etc.) have a structural framework consisting of alternating AlO4 and SiO4 tetrahedra that encompass an empty space called a β-cage (Pauling 1930; Weller 2000). [...]if the sample is heated in air above about 400°C, the photochromic property completely disappears (authors’ unpublished data). [...]the pink to purple colour of some sodalite is stable, which indicates that the photogenerated colour centre can be stabilised in specific, unusual conditions (Fritsch et al. 2023). Chemical analyses were conducted with a Thermo Scientific ARL Quant’X energy-dispersive X-ray fluorescence (EDXRF) spectrometer equipped with a Rh X-ray tube for excitation and a Peltier-cooled silicon-drift detector. Ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectra were recorded in transmission mode over a range of 300–1050 nm using a custom-made four-channel spectrometer equipped with four Peltier-cooled CCD detectors, and 175 W xenon and 150 W halogen light sources directed into a large integrating sphere.

The authors recently examined a 15.8 ct Ethiopian opal (Figure 4) that contained numerous red dendritic particles that each measured up to 70-100 μm in diameter (Figure 5). The Ethiopian origin of the opal was confirmed by its physical properties (slight hydrophane character and rounded columnar structure) and its chemical composition (high Ba concentration; Rondeau et al. 2010). The inclusions were identified by Raman microspectroscopy as cinnabar (HgS). Energy-dispersive X-ray fluorescence (EDXRF) chemical analyses of the top surface of the sample confirmed the presence of Hg and S. Cinnabar and opal are associated in numerous localities, particularly in the western USA (Knopf 1915; Gettens et al. 1972; https://www.mindat.org/min-3004. html). However, cinnabar is only rarely mentioned as inclusions in opal (see, e.g., Gaillou 2015). Also known is an opalised or silicified cinnabar material known as myrickite, in which the high concentration of cinnabar inclusions induces an intense orange or red colour (Manutchehr-Danai 2009; Melero et al. 2019). To the authors' knowledge, this is the first time that cinnabar inclusions have been documented in opal from Ethiopia. Their dendritic habit suggests relatively fast growth, whereas magnetite inclusions reported in Ethiopian opal have a well-formed octahedral habit (Rondeau et al. 2010), suggesting slow growth. This points to a vast domain of parameters possible (at least in terms of growth rate and chemistry) for the formation of inclusions in Ethiopian opal, which perhaps reflects the expansive region over which those deposits occur.

[...]a green rock with a similar cryptocrystalline texture and colour is known as chromocre (N. Meisser, pers. comm. 2023; Figure 2) and comes from Ecouchet, Autun, Saône-et-Loire department, France. [...]based only on visual comparisons to similar material, the samples we procured were likel-chrome-pyrophyllite. [...]in the absence of information on its geological occurrence, we make some assumptions about its formation based on its inclusions and associated minerals. Chemical analyses were conducted with a Thermo Scientific ARL Quant’X energy-dispersive X-ray fluorescence (EDXRF) spectrometer equipped with an Rh X-ray tube for excitation and a Peltier-cooled silicon drift detector. [...]we obtained semi-quantitative EDXRF analyses for 20 samples that were polished on one side.