This is a perfect exemplar of something I really hate about this website. A poster asks for advice about how to buy a diamond, and instead he gets mostly replies saying "don't buy a diamond." I will try and actually be helpful.
Being pedantic, the original question was
I would like to propose to my girlfriend in the near future. For this I would like to use a diamond ring.
and your first suggestion was
Propose with a "fake" ring, then go shopping for the "real" ring together.
This seems like a reasonable suggestion. But I think you applied the same heuristics as others, just less far. Those heuristics being "infer motives from question; give advice satisfying inferred motives". The motive you inferred seems to have been "I'd like to propose, and I'd like my girlfriend to end up with a diamond ring". Others seem to have inferred something closer to "I'd like to propose with a ring with a pretty stone in it".
Basically I think that "being helpful" is a difficult game, and "answer the question as asked" doesn't lead to optimal helpfulness, and I don't have a good solution for this.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.