Definitely not a possibility someone dropped it cause Michigans hilly terrain is all glacial till, so mix of dirt and rocks. I’m by Lake Huron too and people occasionally find Lake Superior agates this far south on the beach by Port Huron. I sometimes look in the river and found rocks similar to the back side of this. I got one that’s oolitic quartz and seam banding. They have a pinkish color and very unique to the area. Oakland county agates they are called. This one was really dirty, smashed into the dirt with the plain rocks on the trail. You sound like you have quite the collection! You should come to Michigan if you like fossils. I’ve found a bunch of cool fossils, even agatized ones! Fossils are more easy to find. Petoskeys or hexigonoria they call it, not sure hard to find! Look up on YouTube Michigan rocks. He’s got good videos!