A unified picture of icosahedral cluster solids obtained by comparing Al-based icosahedral quasicrystals to boron-based cluster solids, which consist of icosahedral clusters of the same group III element, B, is discussed. Metallic-covalent bonding conversion in icosahedral clusters of Al and B was demonstrated by molecular orbital calculations. According to the electron density distribution obtained using the maximum entropy method and the Rietveld method, the bonding conversion phenomenon occurs even in cluster solids such as some Al- and B-based icosahedral approximant phases (Al12Re, α-AlMnSi, α-rhombohedral boron). The covalent bonds observed by the electron density analysis give rise to extremely low carrier density and small density of states at EF in α-AlMnSi and perhaps in icosahedral quasicrystals. In β-rhombohedral boron, the electronic structure and properties depend on slight variation in atomic environment of the icosahedral multiple-shell structure of the crystals. As metallic atoms are doped into specific atomic sites of β-rhombohedral boron, the atomic structure, the electrical conductivity and its temperature dependence approach those of AlLiCu icosahedral quasicrystal and equal those of AlPdRe icosahedral quasicrystal. From the absolute density measurement, it was revealed that the concept of rigid sphere packing, i.e. metallic bonding, is not applicable to AlPdRe icosahedral quasicrystal. Various semiconductor-like behaviors of Al-based icosahedral quasicrystals were demonstrated by means of not only conventional conductivity and Hall coefficient measurements but also modulated photocurrent measurements. In addition, the thermoelectric properties of AlPdRe quasicrystal were estimated. The potential for application of the icosahedral quasicrystals as thermoelectric materials is also discussed.