Silicon has a band gap of 1.2 eV. It is transparent to infrared with lower photon energy, there are no transitions in which those energies can be absorbed. This is the same reason why air and glass are transparent in the visible.
Silicon strongly absorbs (or reflects) photons in the visible part of the spectrum, as those induce electronic transitions of electrons from the valence band to the conduction band.
Transparent substances have a bandgap larger than the photon energy of blue light, larger than about 4 eV. Not all wide-gap insulators are transparent - there are other processes that can absorb light, local transitions.