Moqui Marbles?
Someone posted this idea on the Space.com bbs.
I found these links after a web search:
Moqui Marbles
http://www.lotzorox.com/moqui.html
Moki Marbles, Escalante (#9711)
http://www.utahphotowild.com/small/pages/small4.htm
Valley of Moki Marbles, Escalante (#9712)
http://www.utahphotowild.com/small/pages/small3.htm
As explained on these sites, they consist of a hematite outer layer with a sandstone interior. This would explain why the hematite reading disappeared when the round spheres were crushed by the air bags.
However, these iron concretions seen in the US southwest are quite hard. Perhaps this happens over geological time in an arid climate, and Mars spheres are still forming.
Bob Clark