Symbolism, Imagery, Allegory
We already know that silver is a symbol in its own right (see "Silver" in this section), but Conrad takes things to another level and provides symbols for our favorite precious metal, the people who pursue it, and the power it wields.
Is all the meta talk making your mind hurt? Okay, we'll back up and take you through it. The book opens with a chapter on the landscape of Costaguana, which includes a story about two gringos and a mozo who allegedly stole a donkey and went looking for gold among the rocks of the Azuera, a peninsula on the coast near Sulaco. They were not first to make the attempt; apparently others had tried to find this rumored treasure and died in the process.
According to local legend, the two gringos had remained alive to this day through some magic connected with the treasure and their greed for it:
As to the mozo, a Sulaco man—his wife paid for some masses, and the poor four-footed beast, being without sin, had been probably permitted to die; but the two gringos, spectral and alive, are believed to be dwelling to this day amongst the rocks, under the fatal spell of their success. Their souls cannot tear themselves away from their bodies mounting guard over the discovered treasure. They are now rich and hungry and thirsty—a strange theory of tenacious gringo ghosts suffering in their starved and parched flesh of defiant heretics, where a Christian would have renounced and been released. (I.1.4)
So, you probably get the point—it's pretty easy to connect the dots between this story and the events of the novel, in which Nostromo and Gould (both Europeans) get kind of metaphorically trapped by their obsession with the silver. Nostromo even makes the connection himself when he argues to Martin that their mission with the silver was more dangerous than sending a man into the rocks of Azuera to look for treasure.