We investigate the size of mineral grains of Earth's rocks in computer models of the whole Earth. This is relevant because grain size affects the stiffness (large grains are stiffer) and deformation of the Earth's mantle. We see that mineral grains grow inside stable non-deforming regions of the Earth. However, these regions are less stiff than expected. On the other hand, we find that grain size diminishes during deformation events such as when surface material comes down into the Earth.