The influence of dopants and annealing on the colour stability of ruby

The colour stability of ruby crystals (0.02 wt% Cr) annealed in vacuum, hydrogen, oxygen, and argon, and exposed to an UV irradiation is described. The comparison of samples doped with Cr, Ti, or K to those doped with Cr only is given. The change from the pink colour to brown is attributed to the O− centre formation, which causes an increase of the absorption between 400 to 480 nm. From this point of view the stablest pink colour have rubies containing no further dopant after hydrogen annealing at 1150 °C preceded by an oxygen treatment at 1800 °C. From Ti-doped ruby only those are stable which were annealed in hydrogen or vacuum at 1800 °C.