The idea behind the circuits is definitely to be able to shrink things down. After all, it's these sorts of devices that will power the wands and provide the plugin "chips" to give you special abilities.
I'm still working out exactly how the circuits will work but they will be based on elemental magic energies that behave like light in the circuits. Black marble, sand, and glass will be important components for insulating, combining, and reflecting these energies in various ways.
I'm working from memory, but some of the elemental magics will be things like: life (which is actually a class of energy and each living thing has a slightly different frequency), water, fire, spirit/ethereal, electricity, gravity, light, etc.. And each of those would have different gems or crystals that could be used to generate, remove, or transform magic energies. So maybe you could make a solar powered device that generates water (in fact you will since that's how a player would make their own spring)... or a life force powered device to give you some ability like flying.
The player characters have a special magic aura that NPCs do not possess... because player characters are not from the world. So most NPCs won't be able to use the same magic devices the players will. Well, certainly some will never be usable by a standard NPC.
I change my mind about some of the details sometimes and I'm not sure how flexible I can get without killing the engine. But we'll see. It's all pen and paper dreaming at the moment and may turn out to be impossible as written above... in which case I will fall back on something simpler... though hopefully not as simple as preset recipes.