Elemental and molecular insight into different colors of corundum gemstones

This work aims at deepening the knowledge about corundum colors taking advantage of the availability of a wide set of gem-quality, natural and not treated sapphires of different colors of gemological interest. They are known to display different colors according to the presence and quantity of elements such as chromium, titanium, iron and vanadium acting as chromophores. The most studied varieties of corundum are rubies and blue sapphires, while green, yellow and orange ones of variable shades are much less studied. This work deploys a complementary elemental and molecular approach employing portable X-ray fluorescence (pXRF), photoluminescence (PL) and Raman spectroscopy. It highlights the various answers that can be provided by the involved techniques, such as the higher sensitivity of PL in revealing chromophores with respect to pXRF, and the capability of both to provide clues of differentiation between rubies and pink sapphires. Furthermore, the use of non-invasive techniques that can all be implemented with portable instruments allows to apply this type of study to complex pieces of jewelry, both modern and historical.