Sapphire is a variety of the corundum mineral species, and it is the most highly valued blue colored gemstone in the trade. It is one of the Big 3 colored gems beside Emerald and Ruby. Sapphire consists of Aluminum oxide (Al2O3) with trace amounts of elements such as iron, titanium, chromium, vanadium, or magnesium. The price of the stone depends on the size of the stone, the transparency, the intensity and evenness of the color, the type of inclusions, the treatments and finally the geographic origin.
The analysis of Sapphire starts with taking a photo of it. The general observation is the next step, it is including the weight of the stone which is in carats, the description of the shape and cutting style of the stone checking both the crown and the pavilion.
The measurements are taken using a high-resolution optical measuring system which detects, all the dimensions and angles of the stone automatically in the most accurate way. The transparency is checked by shining the light through the stone. The color which is the combination of hue, tone and saturation is judged using the day light equivalent reflected light. It is also important to inspect if the sapphire displays color change phenomenon or, if cabochon cut, if it shows the asterism phenomenon. Finally in the observation section, it is necessary to determine the specific gravity of the stone using hydrostatic weighing.
Next step is the employment of the Refractometer which is the most important gem-testing instrument. It narrows the number of gemstones in that range of refractive index. Polariscope and Dichroscope are used to determine if the stone is singly refractive (SR) or doubly refractive (DR) and the pleochroism. Although different instruments are used, the conclusion if the stone is natural or synthetic is not yet defined. The Magnification or the gemological microscope is very important to separate natural from synthetic sapphire basing on the types of inclusions which are essential also to determine if the gem is treated or not. The type of inclusions could lead the laboratory to the geographic origin of the stone but to conform that high tech. instruments must be used. Advanced instruments are useful also in case if the stone if free of inclusions because the gemologist cannot judge stone if it is natural or synthetic.
The ED-XRF is the best instrument to discriminate natural and synthetic Sapphire which requires the analysis of the trace-elements. Gallium (Ga) for example is quite relevant in identifying natural Sapphires, being usually absent in synthetic ones.
Almost all sapphires are heated at high to extremely high temperatures to “enhance” their color and clarity. To detect this type of treatment the first analysis is the microscope observation to check if there are some changes occurred to the inclusions inside the gem. The FTIR Spectroscopy provides further information about the presence of heat treatment in the sapphire. It is also performed a UV-Vis/NIR Spectroscopy analysis to check the cutoff line in the spectrum which also helps to understand if the gem is heat treated.
To produce a surface blue color, a lattice diffuse treatment is applied to natural and synthetic colorless sapphires. In this procedure the heat treatment is performed involving titanium and iron oxides which are diffused in the colorless sapphire but only superficially, resulting in a blue layer underneath the surface. It’s a treatment easily recognizable by a careful microscope analysis. An alternative lattice diffusion method, involves very light elements such as Beryllium (Br) which, thanks to their tiny dimensions, are able to cross easily the lattice and change the color of the sapphire till its inner portion. In Sapphire treated with this type of bulk diffusion, the microscope is not enough to prove the treatment without any doubt, also X-Ray fluorescence technique is useless. Only very sensitive chemical analyses instruments such as for example the laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) are able to detect elements like Beryllium.
Fracture filling is another type of enhancement and it is the process of filling in the fissures of a gemstone with different materials. Usually for sapphire glasses containing lead or lead and cobalt are been used. Cobalt is the element responsible for the blue color in sapphires treated in this way. Microscope is a very useful instrument to check for the treatment then supported by an EDXRF analysis.
The geographic origin of the sapphire will effect on its price. If the Sapphires is from Kashmir the price will be much more expensive than the one from Sri Lanka for example. For determining the origin of Sapphire, it is necessary the uses of more than one instrument to get the goal. The determination of the geographic origin is a complex work of synergy of various analysis techniques, using microscope to check the inclusions as starting point. Then micro-Raman spectroscopy is necessary to identify the mineral phases of the crystalline inclusions and thanks to EDXRF are checked the presence and or the ratio of some minerals such as Chromium (Cr), Iron (Fe), or Titanium (Ti). Not less important is the UV-Vis/NIR Spectroscopy by which it is possible to get the geological origin. It is only combining all the data collected by all the instruments that it is possible to reach a reliable opinion about the geographic origin of the sapphire.
GIG uses the standard and advanced equipment to analyze the gemstones to issue a trustable and clear report and has two types of report services: the compact and the extended reports, the type of report depends on the type of the stone. GIG dedicates to the sapphire the extended version of the Colored Stone Report, clearly declaring whether it is natural or synthetic, untreated or treated and, if treated, the type/s of treatment/s. Upon request it is issued also the geographic origin opinion.