Titanium diffusion treatment is known since decades: it can lead to a surface-related blue colour in corundum. Basically, this treatment is a heating process requiring high temperatures in combination with titanium (oxide) as an additive. During the treatment, titanium atoms penetrate into a superficial layer o

by Dr. M.S. Krzemnicki, first published in Facette 26 (May 2020) The talisman of Charlemagne, a gem-bearing reliquary of historic provenance and importance closely associated to the history of Europe (Figure 1), was recently investigated by historians and gemmologists including from SSEF in a collaborative res