So that's clearly not what it is. So what is it?
Top View - about 2" in diameter
Bottom View - there are no openings on the "knob"
I posted these pictures on my Facebook account and guesses include: a door knob, a wheel cap, part of the handle of a utensil, flag pole holder, part of a door bell, part of an old weed wacker, a holder for a curtain rod, an old cap tap, something that covers the end of a screw or nut, or part of an unidentified toy.
I'm leaning towards toy. It's very lightweight. And though it's dirty from all the years underground, it is not corroded or rusty. Thus it is not steel. Instead, it is some lightweight metal, probably brass.
This house has been occupied since the late 19th century, so I am sure many children have lived and played here. I wonder how many years ago it was lost at that spot?
Your opinion is welcome. How about leaving your guess as a comment?