Researchers at Harvard have provided the first experimental demonstration of solid metallic hydrogen. At very high pressures, dihydrogen (H2) is converted to a solid with the properties of an atomic metal, including being reflective.
A popular press summary: https://phys.org/news/2017-01-metallic-hydrogen-theory-reality.html
The original research report in Science magazine: http://science.sciencemag.org/content/early/2017/01/25/science.aal1579/tab-pdf