For details, see:
"The Identification of Blue Diffusion-Treated Sapphires"
"Gems & Gemology," Volume XXVI, Summer 1990, pages 115-133
(Kane, Kammerling, Koivula, Shigley, and Fritsch)
"The diffusion technique, which involves the addition of
chemicals during heat treatment, results in a thin layer of color
at the surface of colorless or light-colored sapphire. The color
was found to be stable to routine cleaning procedures; however,
it may be partially or completely removed if the stone is
repolished or recut."