Diffusion sapphires are those that have been heat-treated with chemicals in a process known as “bulk diffusion” to artificially alter their color.
Often thought of as a blue stone, sapphires are a corundum gem that come in a wide variety of colors, including purple, pinkish-orange, yellow, and green. Red corundum are known as rubies.
Sapphires are often heat-treated to enhance their color. However, some heat treatments also involve the application of beryllium to the stone to shift its hue. Unlike regular heat treatment, this bulk-diffusion process, also called beryllium diffusion, may diminish the value of the stone, though this is less true today than in the past.
Diffusion generally adds yellow tones to a sapphire, causing colorless or green stones to turn yellow or orange, pink stones to turn orange, and dark red or purple stones to turn an orange or deep red. Effects vary on blue stones but often they remain blue.
The process is thought to have originated in Thailand. It works by replacing gaps in the jewel’s molecular structure with the introduced beryllium, according to the Gemological Institute of America.
Whether the stone has been beryllium diffused should be disclosed to any buyer. Initial lack of such disclosures in the Thai gem market during the process's introduction in 2001 created much controversy and some dealers were reluctant to trade in the diffused stones. Today, the practice has become much more accepted in the industry--as long as disclosed--and price differences between diffused and non-diffused stones are shrinking.