Magical Reservoir

Magical energy resides in some form inside all things. Inside a person it is known as a Magical Well, inside an inanimate object it is known as a magical reservoir.
While a magical well is a ball of energy waiting to be drawn out by the person, a reservoir is a series of complex magical matrices that take skill and knowledge to be able to access. Each type of object has a different matrix of energy within, and each requires a different piece of knowledge to access.
A magic user might be very adept at drawing energy from the trees around him or her, but have no ability drawing it from the rocks at their feet. If a magic user finds themselves often needing to draw from reservoirs, then it is in their best interests to learn to do it from a whole host of different forms.
If a magic user is particularly well attuned to an element, then they have a greater success at drawing from things related to their element. For example, the native shamans from the island people of The Itani are nearly always well attuned to water and can draw energy from the waters surrounding their homes much more effectively than almost anyone else. This comes at a cost, however. The same Itani shamans will be nearly completely unable to draw magical energy from fire. Their attunement to water is so strong that it severely limits their access to opposing elements. This is something to always keep in mind if a magic user travels.
When the magical energy of a thing has been totally drained, it will lose its substance in some shape or form. A rock that has been drained will crumble into sand or fine gravel. Water so drained will evaporate. Fire drained will die out. Other elements have different ends, but all with the same result. That element will be gone in both a magical sense and a physical sense.