Diamond is considered to be an insulator,
whereas Se, Si, and Ge are considered to be semiconductors.
Diamond is transparent to visible light, Si and Ge are not;
can be explained with relative band gap energies.
-- optical absorption; energy or photon is absorbed by electron and
-- (Examine wafer of silicon.).
In general, thermal electrons still cannot "jump" bandgap in pure
semiconducting materials; however
Photons of sufficent energy can "promote" conduction electrons.
Photoconductivity -- e.g. Selenium with band gap of 1.8eV