## 3 Analytical Techniques Available
### Complementary Chemical Analyses  
The chemical information is a fundamental test for gemstones, cultural heritage, and archaeological sample identification and characterization. In fact, parallel to the main elements constituting the phases, the particular assemblage of trace elements (i.e., those present and their concentrations) provides a distinctive chemical signature for many gem materials [21, 77]; therefore, different techniques can be considered. The first is Energy Dispersive X-ray Fluorescence (ED-XRF, or only XRF), due to its more straightforward applicability. At the same time, in case the sample characterization requires bypassing the limits of this technique, the chemistry information can be increased based on the Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) and/or Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) application. ED-XRF is a technique whereby a sample is targeted by a high-energy X-ray beam, causing its chemical elements to fluoresce with a spectrum of lower-energy X-rays, with each peak being characteristic of a chemical element. The fluorescent X-ray peak intensities indicate the relative concentrations of the chemical elements. The range of detectable elements goes formally from sodium (Na) to uranium (U), with different detection limits, typically higher for the lighter elements and lower for the heavier ones; nevertheless, the concentration range goes from ppm levels to 100% [78]. The main advantages of this technique are that it does not require any sample preparation and the relative speed of the analysis. Because of all these advantages, ED-XRF has become one of the high-tech routine analyses for gemstone characterization [55]. More recently, Portable XRF (PXRF) instruments have been developed, with many successful applications in archaeometric studies among others. Although the PXRF actually cannot substitute _in toto_ the lab-based counterpart, the possibility of on-site chemical analysis supplies extremely useful information in many archaeological and museum applications [79], even if the detection limits are lower. On the other hand, by SEM-EDS, the chemical information can be combined with the sample morphology at very high magnification; moreover, by EDS, several lighter elements (nominally from boron) can be detected, but this technique generally requires sample preparation. The one exception to this is the Variable Pressure (VP-SEM) or Environmental pressure (E-SEM) application. In fact, by the aqueous vapor-controlled injection in the camera, the metal coating to increase the conductivity of the sample requested by the classic SEM-EDS technique can be bypassed. Nevertheless, quantitative chemical data cannot be collected in VP or E-SEM conditions, but only semi-quantitative information can be acquired [80]. LA-ICP-MS is a micro-destructive technique able to detect almost all the chemical elements with detection limits in the range of parts per million (ppm) to even parts per billion (ppb) levels [64; 81; 82]. A typical gemological instrument set-up requires a single quadrupole ICP-MS. However, several laboratories are equipped with the last generation of a triple-quadrupole instrument, coupled with a 213 nm or 193 nm laser ablation unit. With the high-performance level of the techniques, as well as a considerable work of protocols development performed by the different research teams, the diameter of the ablation holes on the cut and polished stones have been reduced from about 50 \(\upmu\)m to 30 \(\upmu\)m. This fact must be considered a crucial goal for the standard application of the technique on gemstones because usually at least three to five spots are analyzed for each stone, focusing the laser ablation on the girdle. It must be noted that all the chemical techniques previously described require suitable sets of standard samples, not always available and sometimes to be created ad hoc.