IMHO, Illinois is not a great place rocks and minerals.
I grew up in Mattoon, and spent a great deal of time
messing around in gravel pits in nearby. You can find
a lot of fossils in the pits: crinoids, horn corals, etc.,
but I can't remember finding anything decent. Recently,
my father brought home some gravel for the side of his
house. While looking through it I did find a pyritized
fossil, which I haven't bothered identifying yet.
There are some fossil collecting sites in Northern Illinois.
Near Braidwood (very close to Joliet) there is an old
coal mine. Many concretion fossils can be had in this area.
It is my understanding that you need a boat to get to the
best collecting areas. I have collected here and only by foot.
You need permission to hunt here, but it is freely given.
It is managed by Northeastern Illinois University.
The contact is Chris Ledvina. They've recently published a
book about the fossils there.
Another good source of information is ESCONI (the Earth
Science Club of Northern Illinois). They have a number
of fossil nuts there.
I am looking at a rock book called Earth Treasures - The Northeastern
Quadrant for collecting sites near you. It says you can find
"very good" trilobites in quarries near Grafton "with some frequency".
Near Farmington you can find pyritized gastropods at the Midland
Coal strip mine.
You might consider taking here to the Keokuk-Hamilton-Warsaw
area for collecting geodes. You can find geodes in the streams
in the area; there is a stream at the south edge of Hamilton
where you can easily find more geodes than you can carry.
There are a couple of geode "mines" nearby. I have been to
Sheffler's in Alexandria, MO. The geodes at Shefflers were
different than the ones I found in the streams, more quartz
lined and usually prettier.