The word moqui, also commonly spelled moki or mochi, was what the Spaniards called the Hopi Indians of northern Arizona - a name which stuck until almost 1900. Problem was, the word moqui means "the dead" in their language and was, of course, a highly insulting name (typical in European's dealings with the Native Americans).
The Navajo Sandstone is a western rock formation famous to geologists and found primarily in Arizona and Utah. It's the remnant of a huge sand sea desert (an erg) on the western side of the supercontinent of Pangaea around 200 million years ago (early Jurassic Period). Dinosaur trackways are found in some areas of the formation. The coloration and weathering of the Navajo results in some spectacular scenery in northern Arizona and southern Utah as seen below.
Paria Canyon (the "wave")
Checkerboard Mesa, Zion National Park
The Navajo Sandstone is a quartz arenite with about 90% quartz, around 5% potassium feldspar, and 5% clays and other accessory minerals. It's very porous and permeable resulting in easy groundwater flow through the rock unit. Concretion formation is a diagenetic processess - this refers to changes that take place before or during lithification of a sedimentary rock while it's still underground.
Here are the proposed steps in the formation of these concretions (the explanation and figures which follow are based on a paper by Chan, et al., 2005):
1. A small number of detrital grains of iron-bearing silicate minerals (e.g pyroxenes, amphiboles, etc.) accumulated along with the quartz sand which eventually became the Navajo Sandstone.
2. Oxygenated groundwater circulating through the sediments chemically breaks down the Fe-rich minerals and the mobilized iron (Fe3+) then forms hematite (Fe2O3) coatings around the quartz sand grains (microscope view below right). This is what imparts the pink to orange-red color of the Navajo Sandstone.
3. Sometime after burial and cementation of the sediments (lithification), reducing fluids (there's evidence this may be hydrocarbons like methane) from underlying strata move up through the rock heterogeneously on a mm to regional scale (in other words, in some places they do and in others they don't - fluid flow in rock is complex and controlled by differences in porosity and permeability, the orientation and density of fractures, faulting, etc.).
4. This fluid removes the iron oxide films from the quartz grains and "bleaches" the rock from reddish to white. The bleached rock now has pore water with reduced iron (Fe2+).
5. The Fe-rich reducing fluids eventually meet with meteoric groundwater (water derived from the surface) which is oxygenated (oxidizing). At the boundary between the reducing and oxidizing fluids, precipitation of hematite Fe2O3 and goethite FeO(OH) occur and the concretions form (and geology students wonder why they have to take a year of college chemistry!).
6. While commonly spherical, some of the concretions are bulbous, pipe-like, and sheet-like in various areas. There is some evidence that much of this mineralization occurred around 25 million years ago even though the sandstone itself formed much earlier around 200 million years ago (that's not uncommon, diagenetic changes in subsurface rock can occur millions of years after the rock initially lithified).
7. More recent erosion of the sandstone has exposed bedding planes along which large quantities of these concretions (moqui marble) can weather out and be collected (I'm so jealous looking at the picture below, and filled with lust for this collecting locale but since it's now in the Grand Staircase Escalante National Monument, collecting is not allowed).
What I also find interesting is that paper by by Chan, et al., 2005 referenced about is titled Red rock and red planet diagenesis: Comparisons of Earth and Mars concretions. Turns out the so-called "blueberries" discovered by in the Meridiani Planum region by the Mars Rover Opportunity are hematite concretions very similar in form, and possibly origin, to the moqui marbles of Utah!
Mars "blueberries - Rover view and magnified
Moqui marbles are also known as shaman stones or thunderballs. One Hopi legend I heard was that the departed ancestors of the Hopi played games with these "marbles" in the night when spirits are allowed to visit the earth. When the sun rises they must return to the heavens so they leave the marbles behind to let relatives know they are happy and well.
There's a lot of New Age nonsense about moqui marbles (just Google the term to see). I'd like to see some archaeological evidence that shamans of any tribe used them (if anyone has a reference, send it to me). By the way, I paid $5.00 at the mineral show for my two samples which are about 2 inches in diameter. Check out what these greedy bastards are charging for them (I guess mine aren't specially "charged" with woo energy yet).