Thanks for the info, Mike. I'm going to give it a try.
Your question is a hard one to answer because there are probably so many.
It might make a difference what state or area you are searching and how
geologically specific the overlays are.
For example, most knappable rock is called "flint", but there is very little "true" flint
in the states, (allthough I've heard there's some in Ohio
).
Just for starters, I would say for east of the Mississippi it's chert, agate, rhyolite, and quartzite.
Florida is another story.
Out west you have obsidian and jasper to add to the mix.
I don't know if that helps you any.
I'm going to give those overlays a try.
Thanks again, Joe