A treatment that one should be aware and cautious of is diffusion treatment. There is some controversy surrounding this method of treatment. Diffusion treatment consists of exposing the surface of the stone to certain chemicals combined with high heat in a furnace. Generally, only the surface color changes, so if you sliced the gem in half, the inside of the stone would be a different color, usually a paler, less valuable color. The problem is the new surface color is not very deep, only about half a millimeter. If the stone is damaged or perhaps recut, then the original, undesirable color will be noticeable. Beware of this treatment. As of now, the method is used to change the color of ruby, green topaz, and blue sapphire. Usually the original material is a pale or almost colorless stone that is treated to change the color of the surface. Until methods are perfected where the entire stone color is changed, reputable dealers will always tell you which stones have been altered by this method of treatment. Some dealers even refuse to trade in these stones.

However, for star sapphires, this practice is more accepted. GemSelect deals in star sapphires that are diffusion treated. Diffusion treatment is a popular method for treating star sapphire, as the treatment will increase the asterism (the star effect in sapphire and ruby). The diffusion process may reach half a millimeter of depth into the gemstone. The process creates the star effect on the gemstone and the stone can be sold as a diffused star sapphire. The hardness of the gemstone is not affected by the diffusion process and the gemstone therefore keeps its original physical properties. A sapphire made into a diffused star sapphire cannot be polished or recut for the second time as the treatment is only on the surface of the stone. Polishing or recutting would take off this surface treatment. However, most buyers never repolish or recut the gemstones set in jewelry. This is especially true of sapphires, as they are hard and scratch resistant gemstones.