Blue diffusion-treated sapphires are becoming more prevalent than ever before. The diffusion technique, which involves the addition of color-causing 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. Recently, significant quantities of so-called "deep" diffusion-treated sapphires have entered the market. This article documents the properties of these and other blue diffusion-treated sapphires, and presents means of identifying this method using simple gemological tests.