Brown Diamonds Research Articles

Spectroscopic features of brown diamonds from Ural placers

Research subject. Brown diamond crystals from placers of the western slope of the Middle/Northern Urals. Aim. Study of the defect-impurity composition of a sample of brown diamonds from the placers of the western slope of the Middle/ Northern Urals. Methods. In this work, special attention is paid to the analysis of the characteristic spectroscopic features of defects in diamonds from the placers of the western slope of the Middle/Northern Urals using photoluminescence and IR spectroscopy. Results. The studied diamonds from placers of the Middle/Northern Urals are round isometric crystals of a dodecahedral (tetrahexahedral) habitus. According to the results of luminescence (PL) and infrared spectroscopy, three groups of crystals can be distinguished: (i) medium and high nitrogen crystals with low aggregation and dominant defects H3 and 490.7 nm in the PL spectra; (ii) medium and high nitrogen with high aggregation and the predominance of N3 defects in the PL spectra; (iii) low nitrogen with low aggregation and dominant S1 defects in the PL spectra. The first group is characterized by a green glow (excited 405 nm), the second and third groups are characterized by a blue and yellow glow, respectively. The blue glow of the studied crystals is primarily due to the presence of the N3V center. This defect appears as a result of the aggregation sequence of nitrogen impurities and corresponds to diamonds subjected to the highest temperature annealing (1100-1260°C). Diamonds containing the S1 center are characterized by a low content and aggregation of nitrogen compared to crystals of other groups. Group (i) diamond crystals, whose PL spectra are dominated by the H3 and 490.7 nm systems, have a more intense brown color. Conclusions. The data obtained indicate that annealing at higher temperatures is responsible for the formation of N3V centers in highly aggregated diamonds. The low %B1 aggregation and nitrogen content are associated with the presence of a nitrogen getter (presumably titanium) in the S1 center. An intense brown color appears in the group of diamonds with dominant systems A, H3 and 490.7 nm, which indicates a possible relationship between nitrogen and the corresponding optical absorption.

Effect of HPHT annealing in a medium with high hydrogen fugacity on the properties of plastically deformed brown IaAB diamond

To test the possibility of hydrogen diffusion into the dislocation core channels in plastically deformed type IaAB diamonds the diamond sample was annealed at 1500 °C and a pressure of 7.8 GPa for 21 h in a medium with high hydrogen fugacity. The band at 2860 cm−1 corresponding to the CH bonds stretching vibrations appeared in the infrared spectra. It is shown that the 550 nm band in the absorption spectra is due to the formation of complexes with charge transfer from W7 nitrogen centers to dangling bonds in the dislocation cores. Compensation of dangling bonds in dislocation cores with hydrogen led to the destruction of charge-transfer complexes, to a decrease in the intensity of the 550-nm band, and, as a consequence, to a decrease in the color of the diamond crystal.

IR Spectroscopy of Vacancy Clusters (Amber Centers) in CVD Diamonds Nanostructured by Fast Neutron Irradiation

We investigated the IR absorption spectra of CVD diamond damaged by fast neutrons (>0.1 MeV) with high fluences ranging from 1 × 1018 to 2 × 1019 cm−2 and annealed at temperatures of 200 °C to 1680 °C. After annealing above 1000 °C, the formation of “amber-centers” (ACs), associated with multivacancy clusters, is detected as deduced from the appearance of a strong absorption line at 4100 cm−1. Moreover, the concentration of the ACs in the irradiated diamond can be an order of magnitude higher than that observed previously in the darkest brown natural diamonds. A number of other absorption lines, including the H1b center at 4936 cm−1 (0.612 eV) and new lines at ~5700 cm−1 (0.706 eV) and 9320 cm−1 (1.155 eV) not reported before in the literature, are observed, and their intensity evolutions at annealing temperatures are documented. At the highest fluences, all the lines show reduced intensities and broadening and spectral shifts due to a very high defect concentration and partial amorphization. The obtained experimental data can be used for the analysis of defect generation, transformations and healing in irradiated synthetic and natural diamonds.

Люминесценция природных алмазов, индуцированная ультракороткими лазерными импульсами ульрафиолетового диапазона

Обнаружено, что УФ-излучение с длиной волны 257 и 320 nm, не вызывает свечение А-полосы, что связано, по-видимому, с прямым межзонным фотовозбуждением алмаза. In this paper nature crystals of the pink and brown colored diamonds with the evident plastic deformations were comprehensively studied with IR-Fourier spectroscopy and photoluminescence methods. A- and B-centers presence were revealed by IR-Fourier specters. Femtosecond (250 fs) UV-range laser pulses with wavelength of 257, 320, 352 and 365 nm inducted photoluminescence demonstrates N3 and H3 photoluminescence centers and broad structureless A-band luminescence. It was found that UV radiation with a wavelength of 257 and 320 nm does not cause A-band luminescence, which might be due to direct interband photoexcitation of diamond.

Color Genesis of Brown Diamond from the Mengyin Kimberlite, China

The Mengyin diamondiferous kimberlite cluster in Shandong province is one of the three major sources of natural diamond in China, where many brown diamonds are mined, but the genesis of their color is still controversial. In this paper, studies including microscopic examination, optical properties of orthogonal polarization, low temperature photoluminescence spectra, infrared spectra, Raman spectra, ultraviolet-visible absorption spectra, luminescence of cathodoluminescence, and transmission electron microscopy have been carried out on the uncut brown diamonds and their slice samples to constrain on the color genesis of brown diamond from the Mengyin deposit. The results show that the brown color is dominantly caused by plastic deformation, and some samples are also caused by non-deformation-related defects and inclusions.

Optically active centers in brown type IaAB diamonds from the Istok placer in the northeastern Siberian Platform: spectroscopic properties and the effect of HPHT treatment

Optically active centers in brown type IaAB diamonds from the Istok placer in the northeastern Siberian Platform: spectroscopic properties and the effect of HPHT treatment

Unusual optical centres in fancy brown diamonds

Background. Natural brown diamonds with fancy yellow, orange, red and green tints are valuable jewelry raw materials. Their colour is associated with post-crystallisation plastic deformation occurring during transportation from the mantle to the Earth’s surface.Aim. To study point defects in plastically deformed diamonds using optical and IR spectroscopy.Materials and methods. Faceted brown diamonds with fancy tints presented on the Russian market were studied spectroscopically. Selected samples were examined using UV-visible optical absorption spectroscopy and IR spectroscopy.Results. We discovered unusual optical centres in the absorption spectra of the UV-visible region. In addition, the known continuous absorption increasing from the red to the ultraviolet region of the spectrum, as well as the N3 and H3 nitrogen centres and a band at 550 nm, were revealed. In the spectra of yellowish-brown diamonds, the bands at 512.9 nm and 519.9 nm and an accompanying broad band with a maximum of 480 nm were found. The appearance of a yellowish tint of crystals was associated with these optical centres. The bands 506.5 nm, 516.1 nm and 679.7 nm were established in the spectrum of orange-brown diamond samples. An additional absorption continuum associated with single nitrogen atoms and centres (NV)– were observed in greenish-brown diamond samples. Their simultaneous presence causes the appearance of a greenish tint in the diamond colour.Conclusion. The information obtained using IR spectroscopy indicates that an intense brown colour can occur not only in the most common Ia type crystals according to the physical classification of diamonds, but also in relatively rare Ib + IaA type diamonds.

Specifics of high-temperature annealing of brown CVD single crystal diamonds at graphite-stable and diamond-stable conditions

Changes in the set and concentration of optically active defects in brown single-crystal CVD diamonds under high-temperature annealing at low pressure (LPHT) and high pressure (HPHT) were studied by optical spectroscopy. It is established that the pink tint in diamond color, which appears after LPHT annealing and disappears after НРНТ annealing, is caused by NV centers. Transformation of nitrogen–vacancy centers observed in the course of annealing follows the NV → NVN → N3V pattern and is accompanied by a color change from dark brown to practically colorless. At the same time, the absorption spectrum takes the form typical of the IIa + Ib type diamonds with nitrogen concentration at the level of 1–2 ppm. The broadband luminescence present in all CVD diamonds studied is tentatively associated with some extended defects that do not disappear even during HPHT annealing. The set of charge carriers traps is sensitive to annealing parameters, and the high-temperature peak near 661 K appearing after the LPHT annealing on thermoluminescence is considered as promising for dosimetry of ionizing and UV radiations.

Spatial distribution of defects in a plastically deformed natural brown diamond

Photoluminescence, Raman mapping, cathodoluminescence and transmission electron microscopy (TEM) have been carried out on a “zebra” diamond, containing both brown and colourless bands. The stone was cut into two and one part was given high-pressure high temperature (HPHT) treatment, removing the brown colouration. The parts were then cut into (110) sections. In the untreated stone the morphology of brown stripes is consistent with that of slip bands formed during plastic deformation and Raman mapping shows they are under strong compressive stress. Photoluminescence from N3 and H3 centres, as well as lines at 406.3 nm, 491.3 nm and 535.9 nm, are correlated with brown bands in the untreated sample, while cathodoluminescence shows that band-A luminescence is anticorrelated. HPHT treatment reduces internal stress, and eliminates or reduces correlated luminescence. TEM reveals long straight dislocations and dislocation dipoles in the brown bands, consistent with deformation by slip and concurrent intrinsic point defect production, while clear bands have curved and tangled dislocation networks. We postulate that vacancies produced by plastic deformation aggregate into clusters responsible both for the brown colouration and an increase in volume that results in compressive stress. The 535.9 nm line has characteristics of an interstitial-type defect and may be formed by the trapping of interstitials generated during plastic deformation.

Luminescence of brown CVD diamond: 468 nm luminescence center

Detailed study of the luminescence of multiple brown CVD diamonds was performed. It has been found that the well-known optical center with zero-phonon line at 468 nm is a characteristic of brown color. It has been found that the defects responsible for 468 nm center are located within brown striations suggesting close relation of the 468 nm center and the vacancy clusters. Simultaneous reduction of the intensity of 468 nm center and brown color during annealing support the assumption of their close relation. Identical spectroscopic parameters of the 468 nm center and the radiation center with ZPL at 492 nm suggest that the former relates to an intrinsic defect probably containing vacancies. The distribution of intensity of the 468 nm center in some brown diamonds follows the distribution of the NV− center while being opposite to that of the NV0 center and the dislocation-related A-band. This observation suggests the negative charge state of the 468 nm center. Due to its high luminescence efficiency, the 468 nm center can be used as a highly sensitive indicator of the traces of vacancy clusters. We found that the 468 nm center is detected practically in every as-grown CVD diamond including colorless CVD diamonds of high structural perfection and high purity.

Point defects and interstitial climb of 90° partial dislocations in brown type IIa natural diamond

Multiple electron microscopy techniques have been used to study a brown type IIa natural diamond. Electron backscatter diffraction shows evidence of plastic deformation in the form of slip bands, while cathodoluminescence reveals a network of low-angle grain boundaries, also observed in transmission electron microscopy together with long straight dislocations and dislocation dipoles. Aberration-corrected scanning transmission electron microscopy shows interstitial absorption on the 90° partial of both dissociated dislocations and Z-type faulted vacancy dipoles, forming structures similar to that observed in other fcc materials. The observations indicate an interstitial concentration of 1017 to 1019 cm−3 and calculations of point defect concentrations produced by plastic deformation show that this can be produced by strains of the order of 1%. Brown coloration in diamond has been previously attributed to vacancies and vacancy clusters with concentrations around 1018 cm−3, which suggests that roughly equal numbers of interstitials and vacancies are generated in diamond via plastic deformation. Atomic resolution images of Z-type faulted dipoles allowed a stacking fault energy of 472 ± 38 mJ m−2 to be determined.

A Defect Study and Classification of Brown Diamonds with Non-Deformation-Related Color

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.

A Defect Study and Classification of Brown Diamonds with Deformation-Related Color

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.

Spectroscopic evidence of the origin of brown and pink diamonds family from Internatsionalnaya kimberlite pipe (Siberian craton)

New spectroscopic data were obtained to distinguish the specific features of brown and pink diamonds from Internatsionalnaya kimberlite pipe (Siberian craton). It is shown that pink and brown samples differ markedly in the content and degree of aggregation of nitrogen defects. Pink diamonds generally have higher nitrogen content and a lower aggregation state compared to brown samples, which often show significant variations in nitrogen content and aggregation state between different growth zones. The 491 and 576 nm luminescent centres, which are signs of deformed brown diamonds, are absent or of low intensity in pink diamonds implying that high nitrogen content predominantly in A form in the pink diamonds had stiffened the diamonds against natural plastic deformation. The GR1 centre, formed by a neutrally charged vacancy, was observed only in pink diamonds, which may be due to their formation and storage in the mantle at lower-temperature conditions. Mineral inclusions indicate peridotitic and eclogitic paragenesis for studied brown and pink diamonds, respectively. It is suggested that brown diamonds have been formed in a primitive mantle at higher temperatures and/or stored there much longer.

Will bacille Calmette-Guerin immunization arrest the COVID-19 pandemic?

Will bacille Calmette-Guerin immunization arrest the COVID-19 pandemic?

Natural-Color Pink, Purple, Red, and Brown Diamonds: Band of Many Colors

Natural-Color Pink, Purple, Red, and Brown Diamonds: Band of Many Colors

Natural-Color Pink, Purple, Red, and Brown Diamonds: Band of Many Colors

Natural-Color Pink, Purple, Red, and Brown Diamonds: Band of Many Colors

LPHT annealing of brown-to-yellow type Ia diamonds

Low-pressure, high-temperature (LPHT) annealing of yellow-to-brown type Ia natural diamonds was performed to monitor its effects on optical centers within diamond, changes in the observed color, and to assess the process's viability as a commercial gem treatment.With LPHT annealing only, the mostly brown diamonds showed a shift towards yellow coloration; Vis-NIR absorption spectra showed this change was due to a modest increase in H3 intensity. Even at long annealing times (24h at 1800°C) or annealing at high temperatures (2000°C for five minutes), the diamonds did not significantly lose brown coloration. LPHT annealing showed itself as an ineffective means to break apart the vacancy clusters causing the brown color or causing nitrogen disaggregation, which resulted in only a small H3 generation.With LPHT annealing, “amber centers”—a group of several independent bands in the IR between 4200 and 4000cm−1 that disappear with HPHT annealing—were seen to anneal out gradually at various temperatures from 1700 to 2000°C.In contrast, high-pressure, high-temperature (HPHT) annealing effectively removes brown color at similar time/temperature conditions. Without the high stabilizing pressure provided by HPHT annealing techniques, the LPHT annealing showed pronounced damage on inclusions and dramatic surface etching.In subsequent experiments, LPHT annealing was used as a follow-up to laboratory irradiation. The irradiation-related vacancies created greater concentrations of H3 and the vacancy-assisted disaggregation of nitrogen created donors which led to a high concentration of H2 centers. This combination of defects resulted in a pronounced and favorable shift towards saleable yellow colors due to an increase in H3 and a dramatic increase in the H2 center, which led to the suppression of the remaining brownish component.The annealing characteristics for many centers detected by Vis-NIR absorption spectroscopy, FTIR absorption spectroscopy, and photoluminescence spectroscopy were chronicled throughout the study and compared with other LPHT annealing studies and HPHT annealing experiments.

Pink color in Type I diamonds: Is deformation twinning the cause?

Plastic deformation of diamond has long been associated with the generation of color, specifically brown and pink. Extensive previous optical and spectroscopic characterization of natural pink Type I (nitrogen containing) diamonds has revealed two clear groupings, with distinct geographical origins. Group 1 pinks, which have low concentrations of nitrogen and are relatively highly aggregated (IaA ≤ B), have only been found in the Argyle lamproite pipe (Australia) and Santa Elena alluvial deposits (Venezuela). Group 2 pinks, which have much higher nitrogen concentrations and exhibit low levels of aggregation, have been found in deposits from southern Africa, Canada, and Russia. Pink color is intimately associated with deformation lamellae on the {111} crystal planes, and understanding their formation and structure has been a priority with respect to defining the source of this gemologically valuable color center. In group 2 pinks, these {111} lamellae have been characterized as deformation microtwins by both transmission electron microscopy and X‑ray diffraction. Subsequently the {111} lamellae in group 1 pinks have been assumed to also be deformation microtwins. In this paper we report electron backscatter diffraction (EBSD) studies of three brown and six pink naturally deformed diamonds with varying nitrogen concentrations and aggregation states. The results show that there are no deformation microtwins in the group 1 pink or brown diamonds. The study also highlights the usefulness of orientation contrast imaging as a simple and rapid method for determining the presence of microtwins. Our results suggest that the color in the group 1 pink diamonds is not directly related to the presence of deformation twins. However, we propose that twins may have been present but subsequently removed by de-twinning, a process that utilizes the same Shockley partial dislocations involved in the original twinning event. Therefore, it may be the process of twinning (and de-twinning) that creates the defect responsible for pink color, as opposed to the actual structure of microtwins themselves. In addition, a large laboratory data set of pink diamond analyses reveals the occurrence of group 1 pink diamonds in the Namibian marine (secondary) deposits. This would appear to suggest an additional source of group 1 pink diamonds in southern Africa, but the antiquity of these diamonds means that a common source on the former Pangaea supercontinent cannot be ruled out.

Rise from the rough: The growing demands on the brown diamond market

Only 10 years ago, consumer demand for brown diamonds was nearly non-existent. To industry experts, brown diamonds were known as ‘industrial grade diamonds’ or low-grade stones with little value. Today, however, a product that once had very little real and perceived value has become desirable among consumers due to the power of strategic marketing and advertising. By analysing historical and current information on the brown diamond industry, this paper discusses the current challenges and opportunities for these stones.