Marjorie A. Chan, University of Utah
sandstone coloration and numerous iron-oxide concretion "marbles"
(foreground) averaging 2-4 cm in diameter, Jurassic Navajo Sandstone
of southwestern Utah.
Image courtesy of NASA/JPL/Cornell
NASA Mars exploration rover Opportunity's image of hematite "blueberries" (<0.5 cm diameter) from Meridiani Planum, Mars.