Well, it's not a silly fear. I know you're set on Tiffany but bear with me.
When I got engaged to Mr. Bitty, we had had the ring conversation previously. He also "surprised" me. ( I say "surprised" because one shouldn't ask your pre-fiancee to go through your Palm Pilot looking for an address when you have a "LOOK FOR DIAMONDS" appointment front and center!) I had told him I didn't even need a diamond -- that something interesting like a ruby was fine. He insisted because "this is how men are judged" and was dead set on a diamond. He was unemployed at the time and really spent a lot. However, there is no way he would have gotten the quality he did for the same price at Tiffany's. In fact, we talked about Tiffany's. We lived in the Marina in San Francisco (a very yuppie neighborhood) and everyone was getting the Tiffany settings.
What he decided to do is price a good quality LOOSE stone at the carat weight he could afford. It turned out to be a 1.5 carat princess cut with excellent quality. He then went to a jewelry artist, absconded with my favorite ring (a hand me down from Grandma) to show the artist my taste. The setting was perfect, designed specifically for me.
Everyone, and I mean everyone, comments on my ring. They say it is absolutely beautiful. I would never 'trade up' -- even though we could probably afford something bigger now. This ring is evidence how he feels about me. He took the time to design to my taste. That's better than any Tiffany ring I can see on the person next to me in a restaurant. And it cost him MUCH less.
Buy her a Tiffany necklace or rings for her birthdays. Whenever someone dies or an event of personal meaning happens to me, I buy a piece of Tiffany jewelry to remind me of it. If she associates Tiffany with birthdays or Christmas or whatever, I'm pretty sure you'd be off the hook for engagement.