I wonder what the second-order consequences will be. That is, people will still want to signal their commitment/ability to provide when proposing, so the drop in the price of diamonds won't necessarily reduce the overall outlay.

Instead, it would shift the expense to another category (platinum or other expensive metals) or simply lead people to buy more carats (4 is the new 2!). In some ways, these moves could help luxury brands like Tiffany, since their well-known price premium would provide the same signaling device for folks who want to spend tens of thousands of dollars when proposing.