Question

Metals are usually transparent because of their lack of a band gap?

Answer 1

In solids, the atoms cannot usually be considered as isolated. They interact giving rise to bands and band gaps, so the situation is then different from metals, semiconductors and insulators

Answer 2

Band gap is the energy gap . This is one factor in transparency of metal. When band gap is greater than 3 eV and heavy doping is done, metal transmits most radiation and hence is called transparent.

Examples of such metals are tin,zinc and cadmium.

But you must remember that metals with high band gap can also be transparent sometimes.