stromatolite
One of the more interesting rock formations I sell in my shop is stromatolite, aka algae jasper.  A very popular source is the Mary Ellen mine, now underwater.  This past summer a customer took photos of the mine pit and and sent them to me.  I told him what I knew, that my family signed waivers to collect specimens so many years ago.  He happens to have the same surname as our MN U.S. senator and there IS a family connection.