Generally you'll get absorption (opacity) when the photon energy exceeds an energy gap, allowing valence band electrons to be bumped into conduction bands, creating a corresponding hole in the valence. In the case of metals, there is effectively no gap at points in k-space, so there is absorption throughout the spectrum.

A hybrid alumina-silica glass can be created. The hybrid glass has a higher elastic modulus than other silica glass systems, and sheets of it can bend further before fractures form. This glass can be further toughened by creating it with high sodium content, then later exchanging the sodium ions with potassium ions. This type of glass is probably in your pocket right now.

Doped alumina-yttria hybrids are also useful in lasers.