Au13 clusters are considered as elementary building units for the formation of bigger gold nanocrystals. Two concepts for a 13-atom gold cluster have been worked out in order to compensate the mismatch between atomic radius of Au and circumsphere diameter of the icosahedron as limiting atomic distance. The first concept deals with small deviations of the outer shell gold atoms of the icosahedral nanocluster from spherical atomic shape by maintaining the size. The second one proposed a different size of shell atoms in contrast to the central atom. This concept is supported by the recently published experimental evidence for the existence of a body-centered cubic structure of gold at high pressure besides the face-centered one. The density of the gold nanocluster is found to be a little bit smaller than the density of face-centered cubic gold. The elementary icosahedral Au13 cluster may be considered as mediator between BCC and FCC structure. An open question is whether Au13 clusters are present in dense packing structures of molten gold. Applications of Au13 gold clusters are considered.