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Iron
Trace elements in sapphires, such as titanium or chromium, can cause them to appear different colors. Naturally yellow sapphires are often caused by trace amounts of iron in addition to corundum, the main mineral in sapphires. Higher iron concentrations generally result in a more highly saturated yellow sapphire.
Irradiation
Sapphires also can turn a yellow color through low levels of radiation, either naturally or lab-induced. While these irradiated sapphires are not known to cause harm to humans through contact, their color fades over time, especially when exposed to heat and light.
Temperature
Heat-treated yellow sapphires can help dissolve some impurities and can help enhance color and improve clarity. This heat treatment can produce a yellow more vibrant than most natural yellow sapphires. These sapphires, however, are less valuable than comparably saturated, unenhanced sapphires.
Diffusion Treatment
Diffusion treatment is another, more controversial form of heat treatment, in which the element beryllium is added to the yellow sapphire to reduce blue tinges and to produce a more vibrant yellow color. However, because this lab-based process involves changing the composition of the sapphire, sapphires subjected to this method, no matter how vibrant their yellows are, are usually considered much less valuable.