When a demon is defeated, very rarely it will drop a condensed mass of magic essence called a magic stone. This magic stone is something like a concentration of energy, which could be used as a material for a technology unique to this world: spirit engineering.
This magic stone acts as the core for superior demons, and on top of being more beautiful than most precious gems, it is said to contain an unimaginable amount of energy.
As such, the magic stones of superior demons are used as the base for many manufactured goods. Craftsmen are said to even use these as raw materials for the accessories they create. These items will grant various effects upon the wearer, such as an increase in abilities, or an effect that will bind the item to the user only.
High level water magic stones, absorb water from the air, but they are very expensive. Since they need to be frequently replaced, they are rarely used even for military objectives.