Grey Silver powder of unknown composition

Hello all. About 2 years ago, I decided to put 1 stone of corundum, galena, magnetite, uraninite, and garnet schist into a small ziplock bag and carry it around in my bag. It was probably in my backpack for a few months, and at some point, I took it out and left it in my basement. A few days ago, I opened the bag, and it was stained gray. When I dumped out the rocks, a small mass of greyish silver powder with sparkling bits came out. I tried to collect as much of it as possible back into the bag. I touched this with my hand to put the last bits back in. Is this safe, and what would most likely be the composition of this powder?