Hey there EarthGrl! Welcome to the Board!
Good luck with all your rockhounding adventures!
Sounds like you've got a good eye for it, and as you have all ready discovered, there's a lot of research that goes along with it. Out there in your area are a lot of fossils so you might want to get yourself a little fossil handbook. Learning to ID some of the fossils in the field will help you date the area, or dirt or rock, you are looking at.
Pretty cool stuff and will add to your outings.
Anyway you all ready guessed this one right. It's plain to see it's a squarshed, or flattened, petrified honey bee hive, or honeycomb.
Just kidding!
It is a coral fossil. In the large family of Simple Coelenterates, you have the "Tabulates", and this probably a form of "Favosite", more commonly known as "the honeycomb coral". Favosites and related species are widespread in North America and date to the middle Silurian, or about 430 million years ago. They are well exposed in Americas' Heartland along the Ohio and Mississippi Valleys.
Tennessee has Favosites and another coral that looks similar called "Lithostrotionella"which is pretty stuff. And another similar type of coral is "Hexagonaria percarinata", otherwise known as Michigans "Petoskey Stone".
Thank goodness for nicknames like honeycomb coral and Petoskey Stone.
Anyway, there's a few names for you to look up.
Good luck with the hunt!