Thanks for the questions. 

The crystals stay in the possession of the human until they give it to someone. 

The town will likely be attacked a few times, and I was planning on placing the town in a forest where potential dangers might hide. I invite others to feel free to add in a few enemies of their own, but I don't want it getting too out of hand, so this will probably have to be discussed a bit more. 

Collars are actually crafted in town, and humans are encouraged to go get one upon arrival to keep the jewels in. In theory, one could just ignore this and walk around with the jewel somewhere else, but they can be broken fairly easily and the collars are constructed specifically to protect them. The humans are the ones who possess the collars until they give them to someone. 

I guess it is kind of an engagement gift, essentially.