Which elements were among the first observed to become metallic under high pressure?

In the early 1960s, the American chemist Harry G. Drickamer and his colleagues at the University of Illinois at Urbana-Champaign were the first to observe the transformation of materials from insulators to metals under high pressure. Among the first elements observed to undergo this metallization process were iodine, silicon, and germanium.1

These transitions occur because, at sufficient pressure, the forced overlap of electron orbitals induces electron delocalization, a phenomenon consistent with the 1928 prediction by J.D. Bernal that all matter should ultimately become metallic under extreme pressure.1