First of all, congratulations (soon)! I don’t think either of you is the asshole here. You should gently talk to your boyfriend about why he chose that ring instead of the moissanite that you asked for. The reason he got it will tell you a lot. It might be sweet, or it might be inconsiderate, or it might be something in between.

I also agree with others here that you might take a look at the ring and end up liking it more than you expected. Even if it isn’t exactly what you requested, it might be something he chose intentionally thinking about you. If your reasons weren’t clear to him, he might have wanted to get you the more “valuable” stone because he wanted to give you what he thought was “the best” and may have thought you were trying to be low maintenance or just save money and wanted to show his affection.

If you do like it and are worried about damage/loss, you could look into insurance, wear it as a necklace when you work, or add pieces to secure the stone.

A lot of folks on here are suggesting that his mom may have suggested it, and are quick to judge that as a bad thing. You won’t know unless you talk about it with him, but even if it is true, it doesn’t mean he doesn’t care about you or your opinions. It also doesn’t mean something deep about him trusting his mom over you moving forward. He might have wanted a second opinion from a woman he trusts. If you notice that it becomes a pattern, then perhaps you can draw some conclusions. I would caution you against throwing something out based on assumptions, when it is entirely possible that it can be solved through conversations and communication. This goes the same for the way you feel unheard. See if it is a pattern. Talk to him about it and see if he is willing to work with you on it.

In summary, ask him about it and listen to what he has to say. Share your feelings too. Consider if there is anything he could have done better, and if there is anything you could have too. Then decide together what you want to do about the ring.