Disclaiming Banner: [insert witticism] because I do not own ALTA.
Steel
Only the Fire Nation had managed to make true steel. Earth Benders just didn't seem to have the mindset to make iron better than what came out of the ground. It had served them well during the Century War to have steel ships that could freeze in the Poles without sheering apart or was sharp enough to cleave stone.
The steel-makers of the home islands had been 'protected' under the laws of Sozin, Azulon, Ozai, and Zuko. On pain of death the first three Firelords kept the secrets of their war machines (and their secret holders) from their enemies. During the rule of Zuko, metalwork was still a well guarded secret because he had seen his Uncle in chains.