  Production of metallic hydrogen is one of the top three open quests of
physics. Recent low-temperature experiments report different metallization
pressures, varying from 360GPa to 490GPa.
  In this work, we simulate structural properties, vibrational Raman, IR and
optical spectra of hydrogen phase III accounting for proton quantum effects. We
demonstrate that nuclear quantum fluctuations downshift the vibron frequencies
by 25%, introduce a broad line-shape in the Raman spectra, and reduce the
optical gap by 3eV. We show that hydrogen metallization occurs at 380GPa in
phase III due to band overlap, in good agreement with transport data.
  By simulating the optical properties, we predict this state to be a peculiar
black metal, transparent in the IR. The transparent window closes at 450GPa,
but the reflectivity remains low, discarding phase III as the shiny metal
observed at 490GPa. We predict the conductivity onset to increase by 70GPa and
the transparent window to increase by 1.3eV when replacing hydrogen by
deuterium at 0K, underlining that metallization is driven by quantum
fluctuations and is thus isotope dependent.
  We show how hydrogen acquires metallic features (conductivity and brightness)
at different pressures, explaining the apparent contradictions in existing
experimental scenarios.
