Hungry Crystals

While encountering the filled Elemental Crystals is a pretty common occurrence, finding one that is empty, unaffected by even a drop of an Element is quite a rarity. And yet, there is one place, far down in The Depths that seems to be devoid of any of them, where clusters of them can be found.
Compared to the depleted Crystals, which completely lose their properties, becoming quartz structures, these Crystals can be used to gather Elements from any place. Additionally, when they are depleted, they can be reused as many times as one wants, potentially creating almost endless batteries if used in an environment filled with the right Elements. No wonder why the settlements that gather them tend to get pretty rich.

Deadly trip

While Hungry Crystals are so useful and could potentially help many communities, gathering them is a very dangerous job. Crystal Collectors who decide to do so have to watch out for the growing numbers of the creatures the lower they go, and, as there's not really that many maps leading down there, they also need to watch out to not get lost forever in the cave-like corridors.

Storing

After mining them, Hungry Crystals are often stored in special containers that block off all access to the Elements from the outside. While the Elements won't damage them, the "dirty" ones tend to fetch for way lower prices as they first need to be cleaned before selling. They could also be mistaken for the normal Elemental Crystals, so most of the Crystal Collectors prefer to avoid having to argue with their buyers.