Silver

This is the biggie in terms of symbols; in fact, it's so important that it basically becomes its own character. For Nostromo and Charles Gould, the San Tomé silver is like a mistress. For example, Charles's obsession with it leads him to feel guilty for the "infidelity through which his wife was no longer the sole mistress of his thoughts" (III.4.6). Hmm. Maybe he thinks of the silver like this?

Along the same lines, the narrator notices that Nostromo's feelings about the precious metal seem to be a lot more intense than his feelings for Giselle:

He was afraid. He was not afraid of being refused the girl he loved—no mere refusal could stand between him and a woman he desired—but the shining spectre of the treasure rose before him, claiming his allegiance in a silence that could not be gainsaid. (III.12.31)

Okay, fine, you could argue that Nostromo isn't as anxious about keeping Giselle as he is the silver because he's just a confident guy, but that claim is pretty ludicrous; it's clearly more difficult to date your fiancée's sister than it is to hold onto a pile of silver that no one else knows about. Please.

When it's not being compared to a mistress, silver is being compared to a kind of disease. Case in point: Martin Decoud agrees with Nostromo that being entrusted with the silver during the Monterist uprising is a very dangerous proposition: "I can see it well enough myself, that the possession of this treasure is very much like a deadly disease for men situated as we are" (II.7.162).

Why do these guys (and others) find silver so compelling? It's tempting to think greed is the culprit, but we don't think it's as simple as that. For Gould, the silver is tied up with his family's legacy and history; his uncle was killed during one of the country's conflicts, and his father's life was basically ruined when the government gave him the land concession for the mine (which didn't really benefit the elder Gould, since the mine didn't function, and was just a ploy to collect large fees in return for the "favor").

So, for Gould, the silver appears to represent redemption and prosperity. Making the mine lucrative kind of avenges his Dad's failures and, in his mind, will help ensure stability for Costaguana.

With Nostromo, too, keeping the silver is not just about banking some cash (though that's nice, too, of course). Although he's probably trying to make excuses for what is essentially criminal behavior, he does seem to believe (at least somewhat?) that he's righting some kind of injustice by keeping the silver:

"The rich lived on wealth stolen from the people, but he had taken from the rich nothing—nothing that was not lost to them already by their folly and their betrayal. For he had been betrayed—he said—deceived, tempted. […] He had kept the treasure for purposes of revenge; but now he cared nothing for it." (III.12.1053)

Okay, so, we know Nostromo certainly does care about the treasure, so that part is bogus. That said, given his earlier ranting about how the Goulds asked him to protect the silver with very little regard to the risk it entailed (to Nostromo), it seems that he might be legitimately keeping the silver out of revenge for treating him like a supporting player, rather than the hero that he clearly believes he is. To put it differently—and to paraphrase Dirty Dancing—keeping the silver is about saying that nobody puts Baby (Baby Nostromo, that is) in the corner.