I've read about this one before. It would be interesting to see if there were 2 rocks with markings. (cross and turtle) A picture is worth 1000 words. Missouri mainly has "lead"mines but there is some silver also.
I'm from near St. Louis, MO. Almost a couple hours south of St. Louis is a place called Potosi, MO. Not sure without a map how many places Mark Twain National Forest is in but I know part of it is in Potosi. I know cause my family has 80 acres within or near the national forest there. It's so huge and awesome to visit! At any rate here's a link on Potosi. It mentions Moses Austin..https://en.wikipedia.org/wiki/Potosi,_Missouri
He was the father of Stephen F. Austin. He occupied the area prior to going south to Texas with I believe something like 300 families.