## 3 Analytical Techniques Available
### Standard Gemological Tools and Analyses  
In a gemological laboratory, a sample is firstly described based on its morphological characteristics, such as weight, shape and cutting style, dimensions, transparency properties, luster, optical phenomenon (if present), and color. Particularly for the color description, standard daylight is required, and the observation must be conducted in neutral conditions. Following international rules [51; 52; 53], the color is expressed by tone, hue, and saturation in the tridimensional colors system [54].  
Regarding the other standard gemological tests, applicable also to the cultural heritage and archaeological samples, a brief discussion related to the main one is reported below.  
The refracometer is probably the most important standard gem-testing instrument. It is used to establish a gem's refractive index (RI) and birefringence, determine whether the gem is singly refractive (SR) or doubly refractive (DR), and establish what its optic character and optic sign are. Using the refracometer along with the correct technique, the quick identification of a wide range of polished gem materials is possible. The lowest limit for an RI reading is about 1.35, while the highest is determined by the RI of the contact liquid used to perform the analyses (it is usually about 1.80) [55; 56; 57].  
The polariscope can help to determine if a gem is SR, DR, or an aggregate (AGG), and it can also test for optic character or even detect pleochroism [58]. A gem must be transparent (TP) to translucent (TL) for testing with the polariscope; opaque (O) and semi-translucent (STL) stones could give unreliable information.  
The dichroscope is a compact, portable instrument and is equally useful on rough and fashioned gems. It is designed to detect two of the different colors that pleochroic gems show in most viewing directions, and thus helps to confirm whether a gemstone is SR or DR [55; 59].  
Generally, the gemological handheld spectroscopic can provide crucial information to help detect a treatment or identify a synthetic. Indeed, this useful portable small instrument can show the absorption in the visible light spectrum (400-700 nm), correlated with the colors' causes [60]. In addition, it does not require a polished surface, so it can be applied on rough or mounted gem material and also loose gems.  
By the hydrostatic scale, the specific gravity (SG) of a sample can be calculated. To do that, the scale requires a particular accessory consisting of a holder for a beaker of water that sits above, but it does not touch the scale's weighing platform. A wire stand sits directly on the weighing platform and allows an object to be suspended in the beaker for weighing. The SG corresponds to the ratio between the object's weight in air and the weight in water minus the weight in air [55; 61].  
Regarding the stone's hardness, it is not a classical gemological test, due to its micro destructive nature, but it can occasionally be applied, for example, in rough characterization. To perform the Mohs scale test, sets of so-called hardness pencils are commercially available and, courtesy of optical microscopy observation support, it is possible to determine the sample's hardness value [62].