The photochromic behavior of pink diamonds can be explained by competing photoionization processes at multiple defect centers in response to an applied optical pump, according to research by Keal Byrne of the University of Western Australia, Perth, and colleagues. Coloration in diamonds is due to crystalline defects in the crystal lattice, which are also known as color center as they induce color, Byrne explains. The color center responsible for pink coloration is unknown. However, he and his colleagues have modeled the pink diamond photochromic process as 'an optically-driven electronic transition between two (or more) separate defect trap states. They explain that one of these acts as a ground state for the 390 and 550 nanometer absorption bands. Defects introduce energy level transitions into this band gap that absorb invisible frequencies, Byrne explains.