This further supports the idea they are concretions. The "moqui marble" concretions on Earth consist of hematite on the surface with sandstone subsurface.
See also this image of a sandstone formation on Earth that shows layering similar to that at the Opportunity site:

Nick also mentioned that one of the post-RAT images of the outcrop rocks showed it to have a reddish color subsurface. He suggested it was bleached on the surface. This report suggests this happens with sandstone on Earth with the reddish color subsurface being due to hematite and with the bleaching due to reducing agents in waters such as hydrogen sulfide:

Mysteries of Sandstone Colors and Concretions
in Colorado Plateau Canyon Country
By Marjorie A. Chan and William T. Parry
3. Big-Time Bleaching
http://www.ugs.state.ut.us/online/pdf/pi-77.pdf

See also the images of the clustered concretions and partially developed concretions in sandstone in the introduction to the report.

Bob Clark