How to identify natural spinel?

During the Hong Kong Jewelry Exhibition in September 2008, many “Royal Burmese” spinel stones were sold. During post-sale inspection, several stones were suspected of being inauthentic and were sent for testing to SSEF, the Swiss gemological institute. As a result of tests, the stones were recognized as synthetic spinels. That same year, these spinels were offered (and are still actively sold at the Chanthaburi market) in Bangkok during the September jewelry show. The production of synthetic spinels is not new, and the problems of their identification are not new. Nassau describes in his book Gems Made by Man (1980) that the production of synthetic spinels actually began in the mid-18th century when spinel crystals accidentally grew while trying to grow rubies. Recently, Mühlmeister et al. (1993), Schaub (2004) and Notary and Grobon (2004) have well described the properties of such stones. Due to the fact that in recent years a certain amount of red spinel of very good quality has appeared on the modern market with a very attractive price for sellers, it is not surprising that synthetic spinel has again made itself known. Unlike synthetic spinels made using the Verneuil method (they are mostly light blue, green-yellow and colorless), the newly presented samples turned out to be very difficult to distinguish from natural ones. The external resemblance of synthetic spinel to natural spinel of the best quality turned out to be simply amazing. Only careful examination under a microscope and chemical analysis can show the evidence of its synthetic origin. Traditional gemological methods, such as determining the refractive index and absorption spectrum of these flux-grown spinels, are not helpful. The characteristics are absolutely similar to the corresponding characteristics of natural stones. When studying images in a polariscope, these synthetic spinels demonstrate the absence of birefringence, as befits natural spinel, although when using cross filters they show various anomalous transitions characteristic of zones with increased voltage without reference to inclusions. A similar effect can be observed in natural spinels, but only around inclusions. Under ultraviolet LW and HF, synthetic stones show orange-red fluorescence, which is characteristic of natural samples, but along the edges of the ribs a very light milky or yellowish-orange fluorescence can be observed, which is completely unusual for natural spinel. One way or another, all these methods do not seem to be reliable for confidently identifying flux synthetic spinel. Research data under a microscope: All synthetic spinel samples were of exceptional purity. However, all samples contained microscopic tubular cavities containing jagged foreign bodies of a very dark orange or brown color, which were flux residues. All samples showed the presence of gas bubble inclusions formed due to inhomogeneity of flows during cooling. In addition, one of the samples showed, in addition to inclusions of flux residues, the presence of a hexagonal metal inclusion, which owes its origin to the platinum crucible in which the spinel grew. Natural red spinels, especially from Burma (Myanmar), do not differ in purity, often the inclusions in them are very significant, and show healed cracks with a large number of microscopic octahedral crystals with negative optical characteristics, and various crystalline carbonate inclusions with a rounded “alluvial” shape. Often natural Burmese spinel contains brownish iron hydroxide in large cracks and cavities, which should not be confused with the flux residues in synthetic spinels described above. Chemical analysis results: The chemical composition of the analyzed red spinel of synthetic origin is at first glance very similar to the composition of natural red spinel. Unlike Verneuil spinel, which shows a high concentration of aluminum, these synthetic spinels have a stoichiometric Mg:Al ratio and are similar to natural ones. This explains the complete coincidence of the refractive index and density indicators with the corresponding parameters of natural samples. In addition to these main components, differences in the concentrations of chromium (0,5 – 2,5 wt% Cr2O3), in combination with traces of iron, vanadium, nickel, zinc and gallium, were identified in synthetic stones. Platinum was detected in one sample due to the metal flakes mentioned. These elements (except platinum) may also be present in natural spinels. The main distinguishing feature is the low concentration of zinc (0,01 – 0,02 wt% ZnO). The concentration of zinc oxide in natural spinel is usually about 10 times higher (Schaub, 2004). The Raman spectrum shows a distinct broad peak (at 406 nm) for fluxed synthetic spinel, which is absent in the spectrum of natural samples.
Similar peaks are observed in Verneuil spinel. The presence of this peak is apparently explained by deformations in the cubic crystal structure of the stone. In addition, excitation of the samples with a green laser (514 nm) resulted in strong photoluminescence with chromium-related peaks. However, these emission peaks are much less structured than those of natural chromium-containing spinel (Notary and Grobon), which provides another good opportunity to find differences between synthetic and natural spinel. Uncut crystals of flux-grown synthetic spinel also show significant similarities to natural crystals. Like natural ones, they show the triangular shapes of natural edges. Even with careful examination, a specialist buyer of raw crystals is unlikely to be able to notice the differences between the most beautiful synthetic and natural crystals. Only careful examination using a powerful magnifying glass or microscope can help. But the most reliable way to identify whether spinel is natural or synthetic is to contact any serious gemological laboratory that has the necessary equipment for analysis. Is it possible to find out the origin of the spinel in the beads – natural, grown or not spinel at all? Bead size 2mm. Possible tinting. Thank you. Amethysts, 07.04.2020 1- Of course not spinel. Uniform color along the thread. The price of such natural ones is no less than 20 thousand dollars. 2 – It looks like synthetic spinel, but the color matches London topaz. 3 – By measuring the specific gravity in a measuring cup you can easily separate the spinel and London Colors topaz. 4 – If topaz, then this is a great success, since the stone is natural, although highly processed. In addition, necklaces made from London topaz – the most expensive of the topaz “family” – are quite rare on the market. 5 – Made in India from trimmings left over from the cutting industry. 6 – Definitely not glass – the edges are sharp. 7 – If it is topaz, then it was mined colorless in Nigeria (greenish tint). Lyudmila, 07.04.2020 Thank you very much for your detailed answer. I was especially pleased with point 6. Lyudmila, 22.07.2020 Hello dear Admin! Continuing the topic. Is it possible to determine whether it is topaz or spinel using a UV flashlight? I read that spinel changes color under UV rays, but topaz does not, because. does not have luminescence. But this applies only to natural spinel, but I have not found an answer to how its synthetic analogue reacts. I checked the beads (necklace) under a UV flashlight and the color of the stones turned blue. Does this mean something or is UV not helpful in this matter? Thanks for the answer. Amethysts, 22.07.2020 UV radiation is a wonderful assistant for stone lovers. But only . for ultra-fast identification of diamonds in the dark (can be done under the palm of your hand. ). UV radiation will also help determine the impregnation of polymers in turquoise and jadeite when observed through a jadeite filter – Jade Filter. Using UV light to identify colored stones is profanation. In your case: not every spinel changes its color in UV or incandescent lamp light. It all depends on the RATIO of the concentrations of Fe/Cr/Co color centers. So spinel may also not change its color both in UV and in the radiation of a lamp with a tungsten filament. Moreover, the Chelsea filter is also not very suitable, since certain types of blue topaz have a filter reaction in red shades, as well as spinel in blue shades. To accurately recognize spinel or topaz, you need either experience or a refractometer. Quick recognition is easy to do using a polariscope/polarimeter. After all, spinel has an isotropic cubic structure, and topaz has an anisotropic rhombic structure. Lyudmila, 22.07.2020 Previously, you recommended measuring the specific gravity, which would help determine which stone was used in the decoration, but to my great shame, I do not know how to carry out this operation correctly. What attributes will be required besides a measuring cup? Would you be so kind as to explain step by step how this is done? Perhaps this will be interesting not only to me. Amethysts, 22.07.2020 This procedure, including for small objects, is described in detail on the Internet. And of course in the USSR school course of PHYSICS for the 7th grade.

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