We are helping folks out at a Master Gardener clinic, and a client brought us in this "nut." She found it on a sidewalk, and it was the only one she saw. She did not see any shells or notice the leaves on nearby trees. It is solid and firm inside when we cut it open. I initially thought it was a gall of some sort, but now we think it's a nut of some sort, possibly Aesculus? Can anyone give us a hand with this? Thanks!
Oh, for goodness sakes! Thank you for thinking outside the box that we were trapped in! I think you're right. I just rescued an avocado seed from my trash bag and will let it dry to compare. I do believe you're correct! Thank you!