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This is an acorn that I've found in abundance is Rockford, Illinois, USA. I think it may be a red or black oak but I'm not sure. It has a very distinct look with the line showing separating the two halves.

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    Pity its cup is missing - that's one of the things to check for ID...I have to say it doesn't really look like an acorn to me, looks too rounded – Bamboo Apr 19 '18 at 20:45
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    Kinda looks like a walnut.... – Tim Nevins Apr 19 '18 at 21:34
  • Yeah, even without seeing the nut, the tree looks like a walnut (not like an oak). Not sure what hickories are like. I'm not saying it is a walnut, though. – Brōtsyorfuzthrāx May 17 '18 at 4:02
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I'm pretty sure that the nut is one of the less-common hickories - probably bitternut (Carya cordiformis). The nut of this species is very smooth and does not have ridges like other hickories. It also has the same point at the bottom that's in your photo, and appears to have a central line. Here are some photos:

Pignut https://davesgarden.com/guides/pf/showimage/57394/

Bitternut http://www.friendsofeloisebutler.org/pages/plants/hickory_bitternut.html (go to the bottom of the page for the photo)

https://www.alamy.com/stock-photo-bitter-nut-hickory-bitternut-hockory-carya-cordiformis-nuts-with-husks-7321191.html

Lots of hickories https://iythealth.com/hickory-tree-nuts/ (great photo of a number of species at about the middle of the page)

The bitternut's range is correct for you - it's found throughout Illinois and into central Wisconsin.

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    Is the nut you pictured cylindrical or kind of flattened? – Jurp Apr 21 '18 at 0:39
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    Then it's not an acorn: wikihow.com/Identify-Oaks-by-the-Acorns. I realize that wikis aren't great references, but this one seems more correct than most. Note that in Midwest woods, Oaks and Hickories grow together. If your neighborhood was built in an Oak-Hickory woods, then it's likely there are hickories there. Also note that Bitternuts aren't shagbarks. – Jurp Apr 21 '18 at 23:24
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    Shagbark hickories are roughly star-shaped, but bitternuts are flattened, with a central seam, like the one you pictured. Here are some examples; sheffields.com/seeds-for-sale/Carya/cordiformis///////358/… ---- You should be able to add the leaf photos (which I'd love to see!) by editing your original post. The instructions should be in the "Help Center". – Jurp Apr 25 '18 at 11:24
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    Nice pictures. The leaves in the second photo look like filbert/hazelnut (Corylus americana), but the nut does NOT look like your common filbert nut. The nut certainly looks like a member of the walnut family (which includes hickories, of course), but I believe that all members of that family have pinnately compound leaves. Obvious question (and pardon me for asking it), but you're absolutely sure the trees sprouted from the nut you asked about? – Jurp Apr 26 '18 at 0:45
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    After checking some references, the only other possible common species seems to be American Beech, but the nut shape and range is wrong. The Corylus range is correct, at least. I don't suppose that the nut has a "cap scar" on it that wasn't pictured? – Jurp Apr 26 '18 at 1:43

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