On the scale of things, a piece of nice rock isn't such a bad buy. If the partner adores it, why not?
Diamonds aren't worthless once you've bought them, the estate will get some residual value!
which is also a great place to buy diamond rings and other jewlery - auction house estate sales. Their experienced jewelry experts will be truthful about the stone quality etc. And you can get really quality stuff for say, 30% of retail.