Some time in 1892, William Chandler Roberts-Austen, who at the time was Chemist and Assayer of the Royal Mint, presented to the Royal Society a sample of a novel alloy of gold and aluminum he had made. It contained two atoms of aluminum for each atom of gold: Al2Au. It had never been made before in all of human history, since aluminum itself was a recent discovery.
It was bright purple, and I have been intermittently obsessed with it all my adult life.