gems, corundum, color.4

Natural yellow sapphires derive their coloration from either Fe³⁺ or h•–Fe³⁺ chromophores, or a combination of both, with Sri Lankan sapphires typically colored by h•–Fe³⁺ alone, while high-iron sapphires from basaltic terrains like Australia and Thailand may show contributions from both chromophores; the color saturation from Fe³⁺ shows a non-linear relationship with areal density, in contrast to five other natural corundum chromophores, and weakly yellow or colorless sapphires can be intensified through heat treatment that generates h•–Fe³⁺ by altering one of two specific internal chemistries—distinct processes that are discussed—whereas beryllium diffusion can shift donor- to acceptor-type chemistry, facilitating the formation of h• and enhancing yellow color, though synthetic yellow sapphires typically rely on Ni³⁺ or Ni³⁺ plus Cr³⁺ for color, with Czochralski-grown Ni-doped specimens occasionally exhibiting enhanced saturation via heat treatment.