Legend has it that in 1477, a love-struck Austrian, Archduke Maximilian, came up with a gem of an idea: to give his future wife, Mary of Burgundy, a diamond ring to celebrate their imminent engagement. She wore the ring on her third finger on the left hand, the same finger believed by ancient Egyptians to have a vein that led straight to the heart. Others believe diamond betrothal rings were used to signify the eternal love of a couple – seeing as they are the hardest mineral in the world.