I sold a 14k gold engagement ring once and the jeweler didn't care about the diamonds at all, and only was interested in the melt value of the gold. I still have the diamonds because he was literally going to give $0 for them. Unless they are quite large or a rare color etc, I don't think they have much resale value. At a reputable jeweler however you should be able to get 80% or more of the spot price.

The history of diamonds is quite interesting actually: http://www.theatlantic.com/magazine/archive/1982/02/have-you...

tl;dr don't buy jewelry as an investment, unless you own a jewelry store