Cortinarius sp.
YOUNG and mature CORTINARIUS specimens
Even knowledgeable mushroom hunters must be extremely cautious to avoid being tricked by a toxic species of Cortinarius when they thought they were picking an edible Wood Blewit (Lepista nuda).
Both mushrooms have a lot in common--their identical fall woodland habitat and stature, including a plump stem base, plus an attractive violet color.
But all species of Cortinarius have rusty brown spores (the spores of Lepista nuda are lightly pink). Cortinarius species also feature a cobweb-like membrane stretching over the gills when young. As the cap expands and matures, traces of that membrane linger on the stem and the edge of the cap, where falling spores often dust it with their rusty brown color. There are a few violet Cortinarius species in Illinois, and some of them are amazing look-alikes for the Wood Blewit.
Another creepy fact: Both mushrooms commonly grow side-by-side in exactly the same habitat.