  We present an experiment on crystallization of packings of macroscopic
granular spheres. This system is often considered to be a model for thermally
driven atomic or colloidal systems. Cyclically shearing a packing of frictional
spheres, we observe a first order phase transition from a disordered to an
ordered state. The ordered state consists of crystallites of mixed FCC and HCP
symmetry that coexist with the amorphous bulk. The transition, initiated by
homogeneous nucleation, overcomes a barrier at 64.5% volume fraction.
Nucleation consists predominantly of the dissolving of small nuclei and the
growth of nuclei that have reached a critical size of about ten spheres.
