Metamorphic, Biotite Gneiss would be my basic explanation and smarter and more eager person than me might say a whole lot more. If you like rocks there are so many books that have been written about rocks and how they came to be. Each and every rock found on earth has so much that can be said about it and how they came to be. To put it in a nutshell, all rocks I've concluded are the result of the big bang, which I find is so much easier to explain.