We have investigated the structures, stabilities, and electronic properties of diamondoids and analogous compounds composed of heavier group 14 elements, silicon, germanium, and tin. Systematic quantum chemical studies on octahedral and icosahedral structures up to C1820H420, Si-1 H-100(300), Ge600H200, and Sn600H200 were performed to elucidate periodic trends among the group 14 nanostructures. The octahedral diamondoid analogues, which are superimposable with cubic bulk lattice, were found to be favored over the icosahedral structures for each element. However, due to the increasing metallic character of the heavier group 14 elements, the energy difference between the octahedral and icosahedral structures decreases when moving down from carbon to tin. The electronic characteristics of the icosahedral diamondoid analogues suggest them to possess electronic properties different from materials with a cubic bulk lattice. The obtained structural characteristics and periodic trends are expected to be helpful in the characterization of experimentally known group 14 nanocrystals.