The Impenetrable

On his first sight of the Dragon Smaug in Erebor, Bilbo Baggins had noticed his gleaming underbelly of gems. When the chance presented itself on their second encounter, Bilbo flattered the Dragon, calling him 'Lord Smaug the Impenetrable'¹ and admiring his armoured scales, as well as the coating of gems that protected his more vulnerable underparts. Amused by the Hobbit's flattering talk, Smaug displayed his gleaming layer of gems, thus revealing the bare patch in the hollow of his left breast. This discovery proved critical to the events that followed, as Bard the Bowman would later use it to penetrate Smaug's 'Impenetrable' armour and bring the Dragon's life to an end.