“I remember once I asked Maester Luwin why he wore a chain around his throat.”

Maester Aemon touched his own collar lightly, his bony, wrinkled finger stroking the heavy metal links. “Go on.”

“He told me that a maester’s collar is made of chain to remind him that he is sworn to serve,” Jon said, remembering. 
“I asked why each link was a different metal. A silver chain would look much finer with his grey robes, I said. 
A maester 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 realm he serves, isn’t that so? 
Lords are gold and knights 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 metals, and a land needs all sorts of people.”

Maester Aemon smiled. “And so?”

“The Night’s Watch needs all sorts too. Why else have rangers and stewards and builders? Lord Randyll couldn’t make Sam a warrior, and Ser Alliser won’t either. You can’t hammer tin into iron, but that doesn’t mean tin is useless.” 

......................Game of Thrones Book 1 Chapter Jon