The causes of colour variation in Kashan synthetic rubies and pink sapphires

摘要

The trace element patterns of Kashan synthetic rubies and pink sapphires reveal two colour-causing transition metal elements, chromium and titanium, which are present in ranges of concentrations up to 0.23 wt. percent TiO_2 and up to 0.62 wt. percent Cr_2O_3 UV-visible absorption spectra consist of the absorption bands of Cr~(3+) on which the absorption bands of Ti~(3+) are superimposed. The titanium component of the spectra predominantly removes the purplish tint of the ordinary ruby colour and thus the saturated red or even orangey red coloration of the synthetic Kashan corundum material is developed. By heat treatment in air, titanium is oxidized from Ti~(3+) to Ti~(4+) and the influence of titanium on the ruby colour is removed.