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

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.