I'm sorry about the 40% haircut. Any jeweler worth doing business with should refund a ring minus the cost to set it.
Diamonds cannot be used, as there is no such thing as a new one. Certainly they can be damaged but that's totally different.
I figure if you buy something a billion years old and return it a few months later in exactly the same condition, if you can only get 60% back then you dramatically overpaid.
It's a very impressive example of avoiding the sunk cost fallacy, though. I think a lot of people would be unwilling to cop that 'loss', even if they realised it would leave them better off overall. (Or perhaps more realistically, they would be unable to admit to themselves that it would leave them better off.)
After researching, we figured out that we could sell our diamond (we only got about 60% of the original price back from a diamond broker) and buy a twice as large synthetic diamond and still save several thousand $.
There is only upside as far I can tell: it looks better (clarity), is bigger (no can tell the difference between a real and "fake" diamond), has fewer environmental and ethical externalities, and is cheaper.