I've gathered some info on these stones, having interest in them since I live in Indiana. Figured I'd post it and start a dedicated thread for these guys. The following text info was compiled and typed up from a few different old Indiana geological reports from the mid to late 1800's. Quick summary, I find it most interesting that the stones can range from nearly pure white, to bluish gray, to orange with or without whiter swirls, to red. They can have black or brown spots, or not. They can have manganese swirls, or not. They are a type of quite fine-grained sandstone with nearly uniform particle size of on average (in the vast majority of samples tested) about 20 microns - though they can be as small as 10 microns or as large as 32 microns.