In chapter 7 of "The Great Gatsby" Fitzgerald uses the oxymoron "unfamiliar and vaguely recognizable" What is the importance of that oxymoron?
To be precise, I'm not sure it is an oxymoron in context, because the entire phrase is "at once definitely unfamiliar and vaguely recognizable, as if I had only heard it described in words." In other words, Nick hasn't ever seen that expression before, but it seems familiar, like he's heard it described. It's important because it comes so soon after that wonderful line about Daisy's voice being full of money. That's an uncommon and impossible phrase, but Nick recognizes its rightness immediately. By contrast, Gatsby's expression isn't clear to Nick. This shows several things: Gatsby fools himself about himself, but not about Daisy, Nick can recognize insight, but his own is only limited, and so on. (It's also a sign in terms of plot that the source of Gatsby's fortune will be revealed soon.)
To piggyback on that excellent answer, I would like to add the following. "Her voice is full of money" is absurd as a definition, but provocative as a meta-physical description. For Nick, this describes that elusive quality of powerful wealth and irresponsibility that she enjoys. As Tom makes a poorly veiled stab at the true, but illegitimate, source of Gatsby's wealth, Nick finally sees something in Gatsby's face he can identify with. Nick sees a hint of the suspected roughneck, the outsider, the fraud, the pretender, the dreamer, the fearful peasant in Gatsby's face. We all present an outer self that we hope will mask our greatest insecurities. Gatsby is about to have his mask fall in front of the one he desires most.