Both bulk Dysprosium (Dy) and Gadolinium (Gd), rare earth metals, grow in HCP bulk crystal structure. As seen in Figure(a), when Gd is deposited on Graphene, it grows with the expected HCP(0001) crystal structure. The flat top islands have 6-fold symmetry. When Dy is deposited on Graphene (Figure (c)), it grows as FCC(111) deduced from the stacking sequence of incomplete islands.
If the islands were HCP(0001), for islands with incomplete top layers, the stacked top layers should point in alternating directions (Figure(b)). However, for FCC(111), incomplete islands would have the stacked layers at the top point in the same direction (Figure(d)). Islands with stacked triangles pointing upwards or downwards can be explained by the two equivalent "ABCABC" vs. "ACBACB" stackings.
M. T. Hershberger, M. Hupalo, P. A. Thiel, and M. C. Tringides, J. Phys. Condens. Matter 25, 225005 (2013). "Growth of fcc(111) Dy multi-height islands on 6H-SiC(0001) graphene"