THE AUSTRALIAN GEMMOLOGIST | Using Luminescence for Diamond Identification
Using Luminescence for Diamond Identification
The use of ultraviolet (UV) light to stimulate light emissions is a common technique used in mineral identification. Geological museums often include a cabinet showcasing colours of fluorescing minerals under a ‘black light’. In gemmology, fluorescence provides a low-cost method that can assist gemmologists to identify a gem and, more importantly, to determine if it is natural or synthetic or has been treated to enhance its appearance. The need is particularly acute with diamonds where artificial and treated versions have been entering the market undisclosed, having profound value implications.
Figure 1. Typical reactions by diamond to LW and SWUV light for natural and laboratory grown (HPHT and CVD) near-colourless diamonds.
Fluorescence
Fluorescence is a category of photoluminescence where light is absorbed by a material and reradiated at higher wavelengths (lower energies). The exciting radiation can be of any wavelength from X-rays to near infra-red, and materials react differently to different wavelengths. For decades, long wave (LW) and short wave (SW) UV lamps have been used to examine gems with wavelengths of 365nm and 255nm, respectively (Deljanin et al., 2021). Such lamps traditionally comprised filtered germicidal tubes (conventional fluorescent tubes without a phosphor coating) in a housing having an enclosed viewing port, however these days more compact options are available using LEDs and small metal-vapour lamps. These light sources also need filters to improve their effectiveness by removing unwanted wavelengths including visible light that could otherwise mask any weak fluorescence. The fluorescent behaviour of gems to different wavelengths provides a method to determine the identity and origin of some gems. This article will focus on the application of fluorescence to diamond identification.
The intensity of fluorescence of a diamond is a common entry in a diamond report, however such grading was originally used as an added identifier for a stone and later became a factor in pricing. This matter has been contentious and recently the effect on the colour grade has been the subject of research by some major laboratories (Bouman et al., 2018). The main use of fluorescence with diamond is to provide an easy method to determine if a diamond is natural, synthetic or colour treated. A distinct advantage of observing fluorescence or phosphorescence is it can be applied to diamonds of any size, and to those that are mounted, even with a closed back. Absorption spectroscopy does not afford such freedom. Due to these advantages, luminescence is commonly used by commercial systems designed to identify synthetic diamonds, for example, the Sherlock Holmes (Yahuda, Israel), the GV3000 (National Gem Testing Centre, China) and the DSecure (DRC Techno, India).
Figure 2. A compact UV system coupled to a Smartphone examining a collection of diamonds. The fluorescence colours and intensities captured under both LW and SWUV can be compared via an app revealing details about defects present.
Figure 3. A type II natural pink diamond with a weak fluorescence from N3 centres despite their concentration being less than 1ppm.
Natural Diamond
A vast proportion of natural diamonds contain a nitrogen defect (N3) that commonly fluoresces blue under LWUV wavelengths and is nearly inert under SWUV. Synthetic diamonds grown by high pressure high temperature (HPHT) or chemical vapour deposition (CVD) have not experienced the time and temperature conditions to produce N3 centres and typically have defects that are more reactive to SWUV than LWUV (Figure 1). These distinctions are used by dealers as a quick test to screen for synthetic diamonds. An Australian-made device (Gemetrix Jewellery Inspector, 2021) coupled with an app is directed at this need and is shown in Figure 2. When coupled to a smartphone camera, the sensitivity of camera sensors can image very weak fluorescence or phosphorescence that may otherwise be invisible to a human eye.
A diamond may fail to fluoresce on account of a too high concentration of a particular aggregate of nitrogen known as an A-centre. For concentrations of this defect above about 400ppm, a diamond will tend to be inert to UV (Eaton-Magaña et al., 2007). For these diamonds the fluorescence can be less than those that are Type II. Although such Type II diamonds are notionally nitrogen-free, even natural diamonds with a concentration of only 1ppm of nitrogen can fluoresce blue (albeit very weakly) from the trace presence of N3 centres. Figure 3 shows the fluorescence of a diamond that exhibited a very low nitrogen concentration, though it is possible that the diamond may contain some growth layers of higher concentration that are responsible for the emissions. The sensitivity of this reaction demonstrates how effective luminescence is compared to absorption spectroscopy for detecting defects.
Normally, fluorescent reactions in a diamond are observed with the table down, mainly because of ease of placement and to minimise artefacts from internal reflections. The transmittivity of UV within a diamond can vary depending on the wavelength of UV, the defects within a diamond and the homogeneity of the defects, consequently the observed reaction can differ depending on whether a gem is presented table up or table down. In a table up orientation any emissions are likely to be reflected within the stone and present a brighter fluorescence. Figure 4 illustrates an extreme example of the difference between the two orientations under the same viewing conditions. In this case it can be speculated that the central core that intersected the table facet had the UV reactive zones, and that the UV absorption prevented UV from reaching the core when illuminated from the pavilion.
Figure 4. An example of a yellowy-orange diamond (synthetic) appearing brightly fluorescent in a table up orientation (left) and virtually inert table down (right) using LWUV. Figure 5. A segment of a pendant showing lower-cost diamonds that fluoresce yellow and strong blue under LWUV. Photo courtesy of Paul Cassarino and reproduced with permission.
The vast majority (95%) of natural colourless diamonds will normally fluoresce with a blue component and nearly 90% of them will be graded with a strength of ‘faint’ or ‘none’. (Moses et al., 1997). Yellow fluorescence occurs with a few percent of natural near-colourless diamonds and is considered undesirable because of its detrimental effect on colour. This undesirability results in price discounts that can be attractive to consumers, resulting in jewellery pieces that feature mostly yellow or strong fluorescing diamonds, such as in Figure 5.
Deep UV
If very short UV wavelengths are used, below about 230nm, diamonds will not transmit the radiation and all fluorescence will be limited to emissions from a few microns depth at the surface. This limitation enables a clear view of growth sectors as they exhibit different colours and intensities (Figure 6). These growth sectors chart the growth history of a diamond that differs depending on whether it was grown naturally or artificially by either HPHT or CVD (Welbourne et al., 1996). Using deep UV fluorescence is considered the most conclusive test to determine if a diamond has been grown naturally or artificially, however such equipment is relatively expensive and beyond the budget of many smaller laboratories. In some instances, like in Figure 4, the geometric growth sectors can be seen with SWUV.
Figure 6. The growth sectors in these HPHT (left) and CVD (right) diamonds are clearly revealed under deep UV using a DiamondView instrument. Photos courtesy of De Beers and reproduced with permission.
Phosphorescence
Some fluorescence can persist after an illuminating source is extinguished, a phenomenon known as phosphorescence. This effect tends to be more pronounced when using the more energetic SWUV, however even visible light can stimulate the phenomenon and some synthetic diamonds will glow a weak orangey-yellow after exposure to ambient room light (Figure 7).
One of the popular tests for synthetic diamonds is to observe for phosphorescence that can persist for tens of seconds with those diamonds that have been grown by HPHT methods. This is the dominant detection method applied to small diamonds, that on account of their value do not receive the scrutiny of gem-testing laboratories that issue certificates. Consequently, in many instances their origin is undisclosed. Figure 8 shows an example of a ring in which almost all the diamonds are phosphorescing, thereby revealing their artificial nature. Natural diamonds can also phosphoresce, though rarely if they are colourless.
A more sophisticated technique of origin determination involves recording the decay time of any phosphorescence, the half-life of natural diamonds being less than 80ms (De Beers UK Ltd, 2019).
Figure 7. With a dark environment and the sensitivity of phone cameras some very weak phosphorescence that lasts for many minutes is visible with this 0.3ct laboratory grown diamond excited by visible light.
Figure 8. A ring comprising many small diamonds thought to be of natural origin by its owner, however the exhibition of strong phosphorescence after SWUV illumination signifies artificial origin.
Coloured Diamonds
For coloured diamonds, different defects in near-colourless diamonds produce different reactions. Irradiation and heat treatment, applied either individually or in combination, are the recognised means to induce colour changes in diamonds and they will normally produce tell-tale signs in their luminescent behaviour. The exception is those diamonds that are inert in the yellow and blue (and colourless) family, as both natural, synthetic or treated varieties can also be inert. The colour origin of green diamonds that owe their colour to radiation are also unable to be determined using luminescence because the defect responsible may have been produced either naturally or artificially (Breeding et al., 2018).
The chart in Figure 9 illustrates the range of luminescent behaviour exhibited by the different coloured diamonds (Photo-luminescent (fluorescence and phosphorescence) reactions for diamonds and simulants, 2022). It will be noted that besides colour, in some instances a pattern can be observed, especially with HPHT-grown diamonds for which growth sectors observed using deep UV can have a strong influence on defect concentrations.
Figure 9. A chart showing the different luminescence reactions of various coloured diamonds exposed to LW and SWUV light.
Figure 10. A pink diamond fluoresces orange and with excitation from a 405nm laser the spectrum reveals much about the diamond by the presence of an N3 system associated with natural diamonds and by nitrogen-vacancy centres resulting from irradiation and annealing. The H3 and H4 centres are nitrogen aggregates combined with a vacancy.
PL Spectroscopy
Fluorescence is a very sensitive technique to reveal the presence of defects, and a more diagnostic version can be applied by examining emission spectra. Features in such spectra can reveal defect details not visible to the eye. To deliver intense illumination to minerals, lasers are typically used though in principle UV lamp lighting could also be used. Typically, 365nm or 405nm are common laser wavelengths that can excite fluorescence across the visible spectrum; however, for some defects other wavelengths are more effective, such as 532nm in the green or 633nm in the red. At these longer wavelengths broad fluorescence is greatly reduced and some defect features and Raman lines can become more prominent.
Natural diamond has a characteristic photoluminescence (PL) emission peak at 415nm and a broader emission peaking at 450nm. A good example of what a PL spectrum can reveal is shown in Figure 10. It is from a pink diamond and based on its LWUV fluorescent orange colour the diamond could be a ‘Golconda’ pink diamond (Ogden, 2020), a treated synthetic diamond or a treated natural diamond. The spectrum shows N3 centres indicative of natural diamond, but also shows features associated with treatment by irradiation and annealing to produce N-V, H3 and H4 centres. (Wang et al., 2018). It is the N-V associated envelope in the red region that gives such diamonds a strong UV fluorescence as shown in the photo accompanying the spectrum, but it is the presence of the N3, H3 and H4 centres that indicate the diamond is natural as they relate to aggregated nitrogen that is almost exclusively found in natural diamonds.
As with many optical spectroscopic techniques, cooling a specimen with liquid nitrogen enhances the peaks (higher and narrower) and in some instances reveals them when at room temperature they are not apparent.
All images courtesy of the author unless otherwise stated.
Editor’s Note: The author has a commercial interest in Gemetrix and associated gemmological products including the Jewellery Inspector and Inspectrum.
References
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Breeding, C., Eaton-Magaña, S. and Shigley, J., 2018. Natural-color green diamonds: a beautiful conundrum. Gems & Gemology, 54(1), pp.1-27. DOI:10.5741/GEMS.54.1.2.
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Deljanin, B., Collins, A., Zaitsev, A., Lu, T., Vins, V., Chapman, J. and Hainschwang, T., 2021. Diamonds – natural, treated & laboratory-grown. pp.127-129.
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Ogden, J., 2020. The history, heritage and hype behind Golconda diamonds. Gems & Jewellery, 29(3), pp.38-40.
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Wang, M., Shi, G., Yuan, J., Han, W. and Bai, Q., 2018. Spectroscopic Characteristics of Treated-Color Natural Diamonds. Journal of Spectroscopy, 5-6, pp.1-10.DOI:10.1155/2018/8153941.
Welbourne, C., Cooper, C. and Spear, P., 1996. De Beers Natural versus synthetic diamond verification instruments. Gems & Gemology, 32(3), pp.156-169.
