Mong Hsu rubies have been heat treated in air at 1100, 1200, 1300, 1400, 1500 and 1600 degrees C. Their visual appearance and surface analysis (XPS) after each stage of heating have been monitored. The characteristic blue core regions of untreated ruby become slightly faded at 1100 degrees C and completely disappear at temperatures above 1500 degrees C. Trace amounts of Na, Ca, Si and Fe were found on the surface of untreated stones. Ti was first detected after heating to 1100 degrees C. Plots of detected surface concentrations of elements versus temperature show that the highest concentration of Fe occurred at 1300 degrees C while surface concentrations of Ti appeared to show two maxima near 1300 and 1500 degrees C. The results suggest that both the changing oxidation state of Fe2+ to Fe3+ and the diffusion of the Fe and Ti ions with temperature are responsible for the color changes through decreasing Fe2+ to Ti4+ charge transfer. (c) 2006 Elsevier B.V. All rights reserved.