Symbolism, Imagery, Allegory
Silas runs to The Rainbow after he discovers that his pot of gold has been stolen.
Yes, Eliot goes there.
To give her some credit, it's a little more complicated than that. For Silas, gold at first symbolizes the achievement of earthly goals. It literally is a symbol to him rather than an end in itself: "money had stood to him as the symbol of earthly good" (1.2.5). This is kind of an interesting comment on symbols, in fact, since Eliot seems to be exploring what it means when we treat things as symbols rather than as, well, things.
Because eventually gold stops being a symbol for Silas, when it magically transmutes into a real, living girl. As Silas reaches forward to take what he thinks is his lost gold, "instead of the hard coin with the familiar resisting outline, his fingers encountered soft warm curls" (1.12.8). That's when gold taken on more of a symbolic meaning in the novel, as Eppie comes to symbolize Silas's gradual absorption into common life.
This is a little confusing, but, if we're right, it's pretty cool: When gold is a symbol to Silas, it doesn't symbolize much of anything for the novel. But when it stops being a symbol for Silas by becoming Eppie, it starts to be a symbol for the novel, and Eppie (although supposedly a real live human) is more of a symbol than the actual gold. Weird, right? Eliot seems to be working through something about the very nature of literary symbolism and how characters in novels are both symbols and people.