This item was found in a gravel pit near Charlton Park. I did not find it but the gravel pit owner ask me to ID it. What makes this item unusal is not what it is but where it was found. See the next photo for the ID and a photo of the back side of this piece.
Now add your own Michigan gravel pit find!
The last photo of the gypsum is under normal house light. As you can see it is about 9 inchs across.
While the first photo shows the eroded exterior of the mineral the flip side of the mineral is a fresh break off a bigger bolder. You can scratch this mineral with your finger nails - it is a Gypsum.