## 3 Analytical Techniques Available
### Spectroscopies  
Three lines of spectroscopies can be considered, in a complementary way: micro-Raman (and Raman) spectroscopy, micro-Attenuated Total Reflection/Fourier Transform Infrared (micro-ATR/FTIR) or classical ATR and FTIR spectroscopy, and Ultraviolet-Visible-Near-IfraRed (UV-Vis/NIR) spectroscopy. While the first two lines represent very useful techniques for material identification, based on their characteristic vibrational modes, the last one is exceptionally suitable for obtaining detailed information about chromophores elements/groups and/or transitional metals, even if it traces. Below is a brief description of those techniques and their applicability. The \"Raman\" effect was discovered in 1928 by Indian physicist Sir. [PERSON], who observed a faint wavelength shift between a light source (monochromatic) and light returned after the interaction with a sample. In a typical Raman experiment, monochromatic radiation (the laser source) is used to excite the sample. The available laser sources belong to UV, visible, and Near-InfraRed (NIR) spectral regions. Thus, the Raman scattered light will also be in the visible region if visible excitation is used. No energy is lost for the elastically scattered Rayleigh light, while the Raman scattered photons lose some energy corresponding to the specific vibrational modes of the sample [65]. Among the many advantages of this technique, the main ones are: it is a non-destructive technique and does not require any sample preparation; the spectrum is characteristic for every mineral phase, similar to a \"fingerprint\" [66]; the technique proves to be very suitable for material identification, by the comparison with reference spectra [67]; and, finally, the spectroscopes can be coupled with an optical microscope, in order to perform the analysis in a confocal way [68, 69]. The infrared spectrometer uses IR light as a source. These wavelengths can be absorbed, transmitted, or reflected by the analyzed sample. This instrument is particularly suitable for material identification in gemology [70], such as epoxy-resins, oils, plastic, and other substances used for colored gemstone treatments, especially the impregnations for clarity enhancements. They have distinct features in their infrared spectra and can be detected by the spectrometer. For gemstones, cultural heritage, and archaeological samples analyses, the sample preparation required by classical FTIR systems represents a significant limitation. This limit can be overpassed due to the development of several accessories, such as the integration sphere, beam condenser, and ATR [64, 71]. The latest evolution of this technique is represented by the micro-FTIR/ATR system: an integrated module equipped with an optical microscope, able to combine the classic FTIR analyses in spatially performing to the sample's morphology. The main advantages are the increase of spatial resolution and the analysis performed without sample preparation, especially in the case of micro-ATR acquisition mode [71, 72, 73]. The UV-Vis/NIR spectrophotometer uses a narrow beam of light in the electromagnetic spectrum between the UV and the NIR, across the visible wavelengths, as a source. A system of lengths focuses the light beam on the gemstone/sample. Those light wavelengths not absorbed by the sample are transmitted to the detector. UV-Vis/NIR spectroscopy aims to observe and characterize several elements present in the samples (i.e., the chromophore groups, transitions elements, etc.) on the basis of their characteristic absorption bands. Researchers typically use this information to determine the material's cause of color [74]. Although this technique traditionally requires sample preparation, courtesy of the latest generation of accessories (i.e., integration sphere), the instruments allow the analysis of transparent as well as opaque samples without preparation [75]. Moreover, by applying an optical fiber system to the spectroscope, it is possible to obtain an adaptable and portable instrument [76].