Silver
Everything which occurs over the course of the narrative springs forth from the mined silver. The entire economy of the small nation of Costaguana is dependent upon exploiting this wealth of raw material. The greed that infects the characters who lay claim to the precious metal is also the primary driving force of the story. Silver becomes the overarching symbol of the destructive effects of materialism.
Buttons
When Nostromo is being celebrated by the crowds, he asks for a knife from the assemblage. This knife will be used to slice off silver buttons from his uniform which he gives to a girl who walks off with them in a haughty manner. The silver buttons symbolize—as foreshadowing—the potential for corruption of the infamously "incorruptible" Nostromo.
The Carriage
The carriage which Mr. and Mrs. Gould ride throughout the story is juxtaposed against the mules which are the best possible transport for the poor people of the country. The carriage is constantly placed within the context of the economic inequity existing between the tiny ownership class and the enormous working class which they exploit. In this way, the carriage becomes a symbol of the fundamental unfairness of capitalist economic ideology.
Lighthouse
The construction of a lighthouse literally threatens the ability of Nostromo to keep his treasure trove of silver a secret. He is operating under an intensifying shadow of paranoia. In this way, the lighthouse becomes important not just literally, but also as a symbol of Nostromo's guilt and fear that his transgression of his supposed incorruptibility will come to light and be revealed.
Missing Ingots
Nostromo's spiral into corruption and guilt and paranoia begins with the discovery that four ingots of silver are missing from the shipment of gold that he and another man have saved from rebel forces. While he returns home to take care of political business, the other man stays behind to guard it only to succumb to madness and suicide. He weighs down his body so that it will sink to the bottom of the ocean after he shoots himself. The missing ingots symbolize the insanity that greed can create.
