"An international team of scientists have discovered a transparent form of the element sodium (Na). The team, led by Artem Oganov, Professor of Theoretical Crystallography at Stony Brook University, and Yanming Ma, the lead author and professor of physics at Jilin University in China, was able to demonstrate that sodium defies normal physical expectations by going transparent under pressure."
It's always fun to do new things with metals, including turning them transparent. This isn't making transparent metal compounds and calling it "transparent aluminum" either. It's quite remarkable. Apparently, the pressure (~3 Mbar) forces electron localization, making the sodium lose some of the principle characteristics of metals. Abstract is here (doi:10.1038/nature07786) If link is faulty, let me know.