Dude has TONS of red flags.

Fiancee is a civil engineer, yet he does not believe that the ring will not be durable even when she said so. OP Does not spend time to google facts. He does not seem to understand that factually this ring will only last for however much was spent on it. Seems to think that just because his fiancee did not request a diamond, could get her an extremely cheap costume ring - when she specifically requested something she could wear everyday.

Yet still in disbelief and aghast that despite all these details, his fiancee clearly cares enough to care for the ring and ensure its longevity.

Add to that that he chose to vent on reddit despite communicating his concerns to his future wife just like any person in a healthy relationship would do.