What is that mean if guy gave girl them chain?

I've meeting this guy recently. We are
not dating just having fun at my place. He is in the military and he is on the ship now but yesterday he came back to his base for a day and he came over to my place then we spent time together. When he leave my place, he gave me his chain. I was surprised little bit and also wondering like why did he do that? That chain is jus black chain. So I wanna know why he did that and what he thinking about me?
What is that mean if guy gave girl them chain?
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