A high quality diamond with GIA paperwork intact won't lose much value second hand. The problem with most retail rings is that they use low quality diamonds and sell at a large markup based in branding. Its like buying generic PC's at Best Buy. You'll lose a ton on resale even on a lightly used item. Not just because of the retail markup, but because there are thousands of SKU's which makes price discovery inefficient.
Because the person buying it new bought it overpriced. Real quality gemstones do not have a new/used stigma as long as they have the proper paperwork. There is some loss in price if they are set because the market shrinks as people see the new setting, they may have a different setting in mind. But people buy overpriced jewelry for the design or name or because they don't know the industry standards which is simply a GIA or similar certificate.
Ask your significant other if she wants a diamond that was owned by someone else before her.
In all seriousness, do not confuse the price of a rough diamond with the price of a polished diamond at a retail store - there's likely a 100% markup. If you were to trade in used rough diamonds, you'd find the price close to freshly mined rough diamonds (though the profit margin for the cutters/polishers is tiny).
