This is a mineral called Chalcedony, I think its one of the weirder things I've gotten my hands on in a while!
It apparently forms in-between crystals, so it's almost like nature's draft excluder or insulating foam, except its rock-hard and smooth to the touch.
Each piece is completely different too, I'd list some independently, but I'd never be able to know which piece was which afterwards because it depends on the angle you observe it to see the colours.
