Up until “Wonder Woman” #52 (by Robert Kanigher and Harry G. Peter) in 1952, it was never explained what the bracelets were made of, but in that issue, they first revealed “amazonium.” Amazonium was an indestructible metal from her home island that was made stronger by a spell from Aphrodite, and let her bracelets repel everything from bullets to energy blasts. The only things that could damage it were magical weapons. While amazonium sounds pretty awesome, it ceased to exist in the post-Crisis continuity. Now her bracelets are made of an unknown but powerful metal.