My [26F] boyfriend [28M] just gave me an extremely special and valuable gift, but I feel really bad about it.

So my boyfriend isn't going to be here for Christmas so we decided to do our gift exchange yesterday. We decided not to go overboard with each other because we are saving up to put a deposit down, have a few months rent, and some money for some furniture, so that we can get into a nice place together in the summer. We both currently live at home in similar situations, both to be around to help our parents out and to live for a reduced rent to save up. We have been dating for 5 years.
So, we had a 200 dollar budget on eachother. I got him a new gaming headset and a few little trinkets and some swag from a Youtuber he likes, and he got me some T-shirts, a new pair of jeans, and a pair of riding boots (I have a horse). Really normal but awesome gifts. But then, he pulled out one more present for me.
It was a diamond necklace. It belonged to his grandmother who passed away in July. I was a little taken aback because we had agreed to keep it small, but he assured me it was within our rules because he didn't pay for it, he inherited it. He is a smartass (lol)! But, it is such an indescribably special memento to their family that I immediately felt extremely guilty he was giving it to me. He said that he wanted to gift it to me because he intends on spending the rest of his life with me and he couldn't think of anything more special than the necklace to show me that. It was basically almost like a promise ring but necklace form. I know that sounds kind of juvenile or silly, but I'm not sure how else to describe it, it kind of felt like a symbol that he intended on proposing in the next little while. He wanted something really special to show me how he felt.
It's not that I'm ungrateful that he gave it to me or anything, it is drop-dead stunningly beautiful, it's just that I honestly feel like he shouldn't have given it to me just because of the value it holds in their family. I'm worried that he rushed that decision and he might regret it. I'm having a hard time articulating exactly why it makes me feel uncomfortable but to be totally honest it does.
I don't know how to balance not wanting to come across as ungrateful but processing the feelings that I feel about getting this necklace. I would really appreciate some insight into whether or not I'm overthinking this, what I should be reflecting on in my own feelings, or if there's anything I should specifically be telling him about this? I definitely cried a lot and it's the most special and beautiful thing that I now own but it just feels almost wrong. Am I overthinking?
Of COURSE I would absolutely, without a doubt, return it if we broke up. Even if he hit my dog with a car and stole my identity, I would return it to his Mom. It's not that I'm worried about him regretting it if we break up or anything because giving it back would be the first thing I would do, but just overall I don't want anyone to feel any resentment towards me for having this because if I'm being totally honest I didn't even know his Grandma. It feels like such a frustrating situation to try to put into words because I feel like the whole thing makes me look horribly ungrateful. What should I do!?
TL;DR my boyfriend gifted me a beautiful family heirloom for Christmas but I worry that he will regret giving it to somebody that isn't in their direct family.


Source.