In this paper, the growth process and the absorption spectra properties of the Cr4+, Nd3+:YAG crystal are reported. The crystal diameter instability, which occurred just beneath the shoulder, is associated with a nearly sharp change in the crystal color. The effect is described in terms of the internal radiative heat transport through the semitransparent garnet crystal which is highly sensitive to the optical properties of the dopant ions. The color gradient along the crystal is assigned to the charge compensation mechanism almost failed at around the shoulder stage of the process, and the instability is mainly attributed to a significant decrease in the radiative heat transfer within the crystal. The effect of radiative heat transfer, within the crystal and the melt, on the crystallization front shape is numerically investigated to simulate the observed instability. Due to the large segregation coefficient of chromium ions, increasing in the optical thickness of the crystal corresponds to a decrease in that of the melt. It is shown that, both of these variations of optical properties result in a significant decrease in the convexity of the crystal-melt interface. The effect of impurity deposition on the crystal surface was found to lower the critical Reynolds number at which the interface inversion occurs.