I believe it's probably an Agate Basin. The concave base variant of agate basin is relatively common in cental Illinois. The angle at which the sides extend and the lack of fluting would probably be a pretty good indication. I have a similar one a friend of mine found in Eastern Iowa. Very nice piece of Avon Chert also.
Thanks Jeff, SRV and Michael.
The thing on the left is a quarter.
I'll be the one finding it's brother JYD.
Avon chert sure is pretty, Peachy.
I'm going to dads next week, I'll try to remember to take some better pics and update.