I am writing a contemporary fairy tale. Based on various bits of lore, I have used woods such as ash and rowan as substances which can affect fairy magic.
In the first instance, I have a fairy lord using a ash staff presumably to augment the magic he is creating (this is not explicitly stated, but he uses the staff as he casts spells and I had intended the staff for the purpose.) In the next instance, I have the protagonist use an ash stick to break through magical barriers and to destroy magical constructs.
The magic is quite varied, with different fairies weaving different spells. With the wood interaction, quite commonly a fairy will cast some sort of defensive barrier around themselves, which bullets can't penetrate but the ash wood can. Another instance sees a fairy lord control magical light constructs to attack a character, but again the ash wood is able to easily destroy the constructs.
The fairy magic is powered by ambient energy from living beings (plants, animals etc,) drawn into crystals which act as a sort of reservoir. A fairy does not hold a crystal, but rather crystals are positioned throughout the land, so access to energy varies depending on distance from one of these crystals. Blocks can be put in place to disrupt this energy, so there are areas which act as deliberate dead zones. There are two different types of fairy magic seen in the book, with users of type A unable to draw power from type B crystals. Crystals can be transformed from A to B and vice versa through a process (corruption, taint, purification etc.) The same base energy is used for both A and B magic, like energy going into a electromagnet that can have its polarity flipped.
Is there any example of a substance that has a dual effect on a form of energy? By "dual effect" I mean similar to the wood in the story being able to augment the use magic and also dissipate magic energies.
I was thinking of something like electricity and metal, where perhaps the staff acts as a conduit, but the stick later acts more like a lighting rod to dissipate the charge, but I'm not really sure, it doesn't feel completely right to me.