“...They are from the Ordovician formations north east of Central Wisconsin ... They occur in chert that has agglomerated in limestone. The origin of oolites is unclear. They apparently formed around some small particle, either organic or inorganic. These are approximately 0.5mm in diameter and are often interspersed with small areas of agate banding and spherulites. I have some massive pieces that weigh a few kilos - one has amethyst. Fossils are often seen in the matrix...”

according to personal communicatioon with Doug Moore, Stevens Point, Wisconsin.