What is this thing someone found in a creek bed?

On Reddit's r/whatisthisthing, someone posted this photo and said, "Found near a creek, very hard and heavy, but not magnetic. Possibly still metal though."

There doesn't seem to be a consensus yet. Suggestions include:

  • meteorite
  • waste material from an old iron factory
  • old mill ball
  • smallbore civil war-era cannon shot
  • siderite nodule/concretion, or hematite

armoured mud ball

Moqui Marble

The mods have a good rule: "Joke comments will get you banned." It's OK to joke in the comments here on Boing Boing, but I can understand why the mods would have such a rule on their subreddit.