Basically came down to some variation on "I don't ask for a lot (she doesn't), but I want the diamond ring."
As some one newly married, I have observed the following.
1. See <something>. Demand for it.
2. When rationally explained why we can't buy it.
3. Simply state.. "I don't ask for a lot, but I want the <something>."
4. I buy it.
"Great idea, I never thought about it, we really need <something>! Problem is, this particular model of <something> is not the best and will not do <something that something is supposed to do> very well. In fact I remember reading bad reviews about this. But fear not! We'll go home, spend some time reading review sites and will find a really good version of <something>."
This will buy you some time, and might even look that YOU want it, which might even make it less appealing to your other half. Now, find a really expensive model of something, but at such price point that the rationality wins, even in someone who usually does not listen to your arguments (step 2). YMMV, but sometimes this tactic works out quite well.
There are a lot of girls out there you haven't met yet. Go find one who gets you.
Of course if she wants it, it's useless.
I've always wondered how other diamond rings get recycled or removed from the market. Diamonds are forever, after all, so why doesn't the price collapse from being awash in used stones from three or four generations of dead women?
After a certain age is reached though, the stigma seems to go away somewhat. We got a 'vintage' ring (about 100 years old) in the end - but they're hard to find; you won't find them in most chain jewelry stores.
Incidentally, the reason you don't see many used bands is different; the metals are worth so much now that it's more cost effective to melt them down than to resell them at the depressed price the market demands for used jewelry compared to new.
Bear in mind that it is in the jeweller's interest to perpetuate such superstitions as it means they can persuade many customers to buy a new ring instead.
Buying a diamonds as a romantic gesture, the buyer is vulnerable for two reasons:
1) They're unlikely to have much experience of buying jewellery; for many, this is the only occasion.
2) They probably feel very strongly about the gesture; it's very important that they get it right. Overspending pales beside the risk of offending the love of their life and/or subjecting them to negative judgement by their peers.
As a result, it is all too easy for jewellers to intimidate the buyer by telling them exactly what is 'expected' and charging absurd prices for it.
Still, 'real' superstition or manufactured one, we didn't fall for it. I suspect many do though. A web search for "used engagement ring back luck" reveals the superstition is pretty widespread.
[Edit: Not that I mean that many people actually _believe_ that the used status of the ring will affect their marriage - I suspect they're more concerned on how their partner and, perhaps more so, their peers will judge their decision]
De Beers marketing worked on you. It is not that hard to destroy diamonds. Google "burning diamonds" for examples.