Back when I was mostly interested in collecting minerals I purchased this piece as it came from a zinc mine near Schullsburg, Wisconsin. It is a piece of shale with two Sowerbyella brachiopods on it and they have an odd yellow color to them.
This one is a complete shell with both valves (or maybe it's just one that was flipped over?).
The other is a partial of the brachial valve and you can see the muscle attachment scars.
Sowerbyella is an Ordovician genera but if anyone has any more information on where they may originate, formation wise, please let me know.