what does gatsby reply when nick asks him how he makes his money? why does nick find that significant?

In the fourth chapter of The Great Gatsby Nick and Gatsby discuss Gatsby's background. Gatsby tells Nick that he came into money when his parents died.

Nick's response is to be skeptical, but Gatsby shows Nick a photo from the Oxford, proving part of his story, then shows him a medal from Montenegro, proving another part of the story.

Nick is convinced, for the moment, that Gatsby's story is mostly true. and he begins to see Gatsby as something of a tragic figure, describing an imagined moment:

"I saw him opening a chest of rubies to ease, wtih their crimson lighted depths, the gnawings of his broken heart."