What is a bubbly dark mineral rock with areas that look like rust, but is not magnetic?

I was walking down by some railroad tracks and a swamp. Down by the swamp I found a bubbly textured rock. Parts of it looked rusty and parts looks very shiny, colorful and smooth. I held a magnet up to it and it didn't stick. I also Think it might be some type of copper ore or something. Anyone help solve this mystery for/with me??