Talked the idea over with Kindred, about the necklaces in my post. Figured it could be a good way for Zac to know if anyone needs his help, so kinda just shoehorned in the idea that he had already been given one earlier. The necklaces specifics are:

Focusing on a crystal and thinking about someone gives you a glimpse of them, where they are, what they are doing, and how they are feeling, so long as they are wearing the necklace. It doesn't let them communicate, or clearly see what is happening. For anyone who has seen Harry Potter, think of it like the mirror shard he had near the end. 

They can use it to contact each other 'somewhat'. In that, if any of them are in desperate need of help, it can relay that feeling to others. AKA, distress can trigger Zac's crystal to let him know someone is in trouble. That way he doesn't have to sit there checking in on each and every person every few minutes.

Otherwise the Crystal traits are:

Heartless hate the things. They're pure crystalized light. 

Constantly playing a song, faint enough not to be heard when worn normally but heard clearly when focused on. It's the sound of Morgan's heart. Anyone familiar with hearts would know, no, it's not the normal 'sound' of a heart. 

The light it gives off can create calmness and basically destress a person. Only when held in their hand and focused on though. So it's not going to change how anyone acts or anything.

It also acts as a constant heal over time, but just barely. More like speed up recovery, rather than something that could directly help in battle. That being said...

Morgan can channel healing through it if she feels someone needs it. It's just that doing so requires ridiculously more power.

These effects are a good bit stronger on Lucian. Something something Prince of Heart. 

Ultimately: Do whatever you want with them. Ignore them, use them as tools against the Heartless. If you think you can do something fun or neat with them, go for it.