"I ask why each link was a different metal. A silver chain would look much finer with his grey robes, I said. Maester Luwin laughed. A master forges his chain with study, he told me. The different metals are each a different kind of learning, gold for the study of money and accounts, silver for healing, iron for warcraft. And he said there were other meanings as well. The collar is supposed to remind a maester of the realms he serves, isn’t that so? Lords are gold and knight steel, but two links can’t make a chain. You also need silver and iron and lead, tin and copper and bronze and all the rest, and those are farmers and smiths and merchants and the like. A chain needs all sorts of metal, and a land needs all sorts of people."
Jon Snow, A Game of Thrones, P. 450