appearance will be a good ruby red. In this way,many attractive red rubies can be produced fromstones with a strong purple component (figure 19).In principle, these stones can be detected easilyby observing the yellow rim in immersion.However, it is conceptually possible to create thesame visual effect without producing a yellow surface-conformal layer. After processing such rubieswith full Be penetration, so they turn orange, wecan reduce saturation of the yellow trapped-holecolor component by heating the stones at a loweroxygen partial pressure. Reducing the oxygen partial pressure will reduce the yellow coloration.Thus, by “tuning” the oxygen partial pressure in aheat-treatment process that takes place after the Bediffusion process, an optimal ruby color may beachieved. This full-depth diffusion followed by anoxygen partial pressure post-process could also beused with our pink sapphire above to achieve padparadscha coloration without a visible diffusionlayer. Unfortunately, rubies and padparadscha-colored sapphires produced by this two-step methodwill be more difficult to detect.It is also useful to consider the color modifications resulting from partial Be diffusion into a bluesapphire where [Ti4+] > [Mg2+]. As we have alreadydiscussed, uniform Be diffusion into such stonesconverts them from blue to yellow. However, agreen overall color appearance results when theinward diffusion of beryllium converts only the outermost layer from blue to yellow. If this green stonewere to be heat treated subsequently in a reducingatmosphere, it would be blue with a colorless outerlayer (figure 20).