This piece was collected from a mixed exposure of Middle Silurian aged Louisville Limestone and lower Devonian aged Jeffersonville Limestone near Louisville, KY. It was found loose within the heavily eroded red clay that often fills gaps within the limestone. The fact that it was loose and rounded made me think it could be a sponge or algae of some sort. Though it could just as easily be a rounded blob of quartz replaced limestone.
Thanks to the comment below, I looked at the specimen again and believe it to be an example of the sponge Hindia sp.