Causes of Colour in Untreated, Heat Treated and Diffusion Treated Orange and Pinkish Orange Sapphires

Colour and causes of colour in untreated, heat treated and beryllium diffusion treated sapphires in the yellow to red colour range are summarized. An overview about the major types of colour centres in sapphire is given. Properties of thermally instable "yellow" and "orange" colour centres are compared with characteristic features of thermally stable "orange" colour centres which are produced by high temperature heat treatment or by beryllium diffusion treatment. The models for understanding the formation of orange colour centres according to trace element contents of the samples as suggested by Häger (2001) and Emmett et al (2003) are described and discussed in detail. Both models are based on trace element ratios in the Mg—Fe—Ti and in the (Be+Mg) —Fe—Ti triangular diagram. The causes of colour of untreated and heat treated padparadscha from Sri Lanka are compared with the colouration of beryllium diffusion treated samples from Ilakaka, Madagascar, and from Songea, Tanzania. The great colour variability of untreated and treated samples is due to the large variation of iron and chromium contents of the samples as well as to the concentration of colour centres.