What Causes Gray And Silver Color Stool?

My granddaughter is 11 months old. Today she had sort of darkish gray and silvery stool, Never seen anything like it before. Have done a little on line reading and seen everything from dairy to uncooked bananas to a switch in formula to bile duct obstruction. I have given her quite a bit of uncooked banana in the past few days. No switch in formula. Also, much of it looked like pebbles. Ideas?