Natural and lab-grown minerals: the beryl case

The distinction between natural and synthetic gemstones is one of the most relevant issue in the gemological field, due to the need of protecting the integrity of natural gems respect to the synthetic ones. Innovations, in the last two decades, have been enormous and pervasive, in particular in the field of synthesis, allowing new synthetic productions, with significant impact on the trade. In fact, a variety of synthetic gems (e.g., diamond, corundum, beryl, quartz, spinel, crysoberyl and opal), obtained by different synthetic processes, such as, flux-growth, hydrothermal synthesis, Verneuil and Czochralski techniques, and, in the case of diamond, HPHT (high pressure and high temperature) and CVD (chemical vapor deposition) processes, are permanently present on the market. Synthetic beryl represents a relevant synthetic gemstone, in terms of quantity produced and availability. A remarkable number of synthetics, mainly emerald, but also aquamarine and different colored (e.g., pink, orange, red, purple) samples, grown either by the flux method or hydrothermal process, are available on the market. The different colorations are obtained using various chromophoric dopants, such as, vanadium, chromium, manganese, iron, cobalt, nickel, copper, also together. The research of diagnostic markers is essential to fully characterize and identify the various natural and synthetic beryls and it is possible using a multimethodological approach, resulting in a combination of standard gemological testing and advanced analytical techniques, mainly non destructive, such as chemical and spectroscopic methods. The following features, obtained by various analytical techniques, provide means to distinguish between various colored natural and synthetic beryls. 1. Gemological properties. Refractive indices, birefringence, specific gravity and UV fluorescence in a few cases can be different between natural and synthetic beryl. 2. Microscopic features. The study of inclusions at the optical microscope can provide important information for the separation of the synthetic beryl from the natural counterpart. In particular, in most cases, synthetic beryl exhibits diagnostic internal features, consisting of strongly inhomogeneous growth structures, residues of seed plates, metallic and flux inclusions, and phenacite crystals. These inclusions prove its artificial nature and allow for a rapid identification. 3. Chemical composition. The various natural and synthetic beryls can be separated on the basis of their chemical composition. Minor and trace elements, such as alkaline and terrous-alkaline elements (Na, Li, K, Mg, Ca, Cs..), chomophore elements (V, Cr, Mn, Fe, Co, Ni, Cu,…), chlorine and metallic elements (Ni, Mo, Pt, Rh,..), are differently present in natural and synthetic stones, allowing for a rapid distinction, in most cases. 4. Spectroscopic features. Spectroscopic techniques are an useful tool in the analysis and identification of the different natural and synthetic beryls. UV-Vis-NIR spectroscopy provides information about the causes of colors, which can be different in natural and synthetic stones, highlighting the presence of “exotic” chromophore ions (e.g. Cu, Ni, Co), typical of some synthetics. Mid-infrared spectroscopy is useful to determine the presence of water molecules, besides to impurities of chlorine and ammonium, which are often relevant in the distinction between natural and synthetic beryl. Moreover, in some cases, this technique can be diagnostic if one is able to establish the nature and the orientation of water molecules (H2O types) in the structural channels of beryl.