A serious and somewhat foolproof answer that should work for most reasonable couples: Buy her a simple silver or gold ring when you propose. This shows commitment, and both silver and gold jewelry hold value far better than diamonds.

After proposing, you have several options:

* If she wants a diamond or moissanite or CZ or other ring, the two of you can go shopping for it later, or commission a custom one.

* If she doesn't want an expensive ring, you can spend it on other things.

* If she gets extremely upset and refuses just because you should have gotten a diamond ring the first time, you might want to rethink whether you want to spend your life with this person.