I happened to come upon this old thread while doing an Internet search on concretions and nodules.
I found it very informative.
DesertUSA for funding this website, and for maintaining this old thread for all of these years.
Roc2rol for asking the original question, and to all of those who replied with insightful comments. It got me to research more into "iron ore" and "concretions". The only thing that I could add is, scientists now know very well the exact chemical reactions that form Moqui Marbles and other similar nodules. I'll leave it to the reader to research on their own, because I found it to be very interesting.