In my story Genii's magic comes from an ancient stone that shatters and embeds into his palm. It's set in a (medieval) world without magic, so it'll be interesting to see how all the other characters react - he's been banished from his village thus far. So not a great reception.
(Initially) I'm portraying the magic as pure energy. It's difficult to control and contain - especially as he's young, quick tempered and stubborn. The energy (in the stone) gradually builds up until it reaches maximum capacity/charge. If the energy isn't expelled (or used) somehow then the stone will eventually start to overload and become unstable (rendering Gen mentally & physically unstable too). It glows, crackles, burns and is "loud" as Genii says - which is really a subconscious warning to do something about it. He doesn't know any of that yet though, so he's "holding it in". smaaaart
Eventually he's going to start to figure it out and use it. And there are a lot of amazing things he can do with it too, but he has a LOT to learn (mostly self confidence) and soooo many obstacles in his way. It's going to be really, really, really tough (because I am a terrible person).
And this is just the first stone - the "Soul Stone". Sorry Gen!
Visually - it's blue and shiny!