Imal points us to: Apparently "forever" has been over-rated – Yahoo! News. This study showed that 75% of women in a survey would rather have a plasma tv than a diamond necklace.
While the author focuses on how this means women are more comfortable with technology, I think it means that diamond necklaces are a lot less remarkable than they used to be.
Diamonds have no intrinsic value, just the totem value that comes from scarcity and social esteem. When those start to fade, the necklace itself is worth a lot less.