Either a meteorite (iron-nickel) or magnetite (Fe2O3, a natural mineral), or scrap steel. Looking at the polished sides it looks to me like it wouldn't be magnetite, but I can't rule it out since it might be stream tumbled. There are places you can send it to have it tested to see if it is indeed a meteorite. I'll have to do some research, its been years since I've looked at those websites. Might be faster if you do a web search on meteorite identification.