Diffusion treatment is often used to change the color of rubies and sapphires, and can be divided into surface diffusion and bulk diffusion.
Surface diffusion treatment is an improvement on the traditional heat treatment method, that is, adding color-causing compounds to the crucible, and treating colorless sapphires that cannot be improved by traditional heat treatment into colored sapphires. However, these transition elements cannot be deeply diffused into the gem's color, which is limited to the surface of the gem or is relatively concentrated at the cracks, and excessive polishing will remove the surface color.
This method can be viewed by immersing oil in oil to see colors concentrated at the edges of the facet, or using EDXRF to detect additional color-causing elements.
Bulk diffusion is also known as deep diffusion treatment or Be diffusion. The Be diffusion treatment is mainly to make colorless or near-colorless corundum appear in the same color as Papalacha. It is also possible to reduce the blue color of dark sapphire to produce a slight yellow color, and then under reducing conditions and then heat treatment can deepen the blue, so that the final blue is more ideal.
Be diffusion treated sapphires
Image from Unusually High Beryllium in Three Blue Sapphires
The natural crystals inside the sapphire exhibit heat-treated characteristics
Image from Unusual Be-Diffused Pink Sapphire . . . A Cautionary Note》
Be diffusion-treated sapphires can be detected using laser-induced breakdown spectrometer (LIBS), laser-etched plasma mass spectrometer LA-ICP-MS, and secondary ion mass spectrometer SIMS.
Resources:
[1] Lu, R; Shen, AH. Unusually High Beryllium in Three Blue Sapphires. GEMS and GEMOLOGY. 2011. 47(3): 232-233.
[2] Renfro, N; Cooper, A. Unusual Be-Diffused Pink Sapphire ... A Cautionary Note. GEMS and GEMOLOGY. 2011. 47(3): 232.
Yu Xiaoyan. Colored Gemology Tutorial.