It's not a Lithograph stone...but perhaps one for some other printing method...
What's it made of? Is it stone?
Lithograph is done on the principal that oil and water don't mix, this is a srone with an image cut in...like a wood cut perhaps...
It's neat!!

What a great find!

Copper plate. maybe. That was the typical method of offset printing in the old days. They'd engrave a copper plate mounted to a wood backer, so it was the same thickness as the rest of the type, and it would get blocked into the page of newspaper type and printed off. I have a couple of these, much smaller, though.
Litho stones were used, re-ground, and used again until they were too thin to be ground down again. They could be nearly any thickness since they were used by themselves, not in a rack of type.
Casey

Thanks everyone! I'm not sure what it's made of. That is part of what I'm dying to know. It has a couple of chips in it where it looks sort of shiny and metallic. It's not magnetic. I don't know if these pictures might help.