I found my diamond! Reto called the jewellery shop people and they said there was nothing they would do for me (apart from replace the stone at my expense), so I decided the time had come to actually look for the original. A quick minute or so of sweeping found it. It was on the floor under our toilet. I should have tried that days ago, but the potential for not finding it was a bit discouraging.
Hurray for my reputation as the finder of things remaining unsullied (incidentally, I also found the lost beanie in that post). And hurray for my marriage not falling apart (symbolically or otherwise)!