Sapphire Color Formation
The evidence of two or more possible causes for the blue colour of sapphire, the colour of most results from a combination of iron and titanium, where they have replaced aluminium in the corundum structure. Intervalence charge transfer involves Fe2_ _ Ti4__ Ti3_ _ Ti3_. Understanding this and related processes is essential for those intending to alter the colour of corundum by heating.
Yellow sapphire may be coloured by one of a number of operations and it is not always possible, in general testing at least, to identify which process has coloured a particular specimen. Nassau (1994) has identified seven different ways in which natural and synthetic yellow sapphires can be coloured; some operations produce unstable colour but only a fade test can establish which stones possess this! Purple and mauve sapphires usually contain some chromium. Black-appearing corundum is usually dark brown from profuse inclusions.