Moqui marbles, also known as “shaman stones,” https://geology.utah.gov/map-pub/survey-notes/glad-you-asked/moqui-marbles/ are spherical or oval-shaped concretions composed primarily of iron oxide and sandstone. These unique geological formations are found in the Navajo Sandstone, primarily in the desert regions of the American Southwest, including Utah, Arizona, and Nevada. Geologically, they are classified as a type of “ironstone concretion.”
The formation process of moqui marbles involves the gradual accumulation of iron-rich minerals around a central core of sandstone. Over millions of years, groundwater rich in iron oxide minerals, such as hematite and goethite, infiltrates the porous sandstone. The iron oxide precipitates out of the groundwater and coats the sand grains, eventually hardening into a dense, metallic outer shell, while the inner sandstone core remains relatively softer and more porous.
The exterior of moqui marbles is typically smooth and glossy, often ranging in color from deep reddish-brown to almost black due to the high concentration of iron oxide. In contrast, the interior is usually lighter in color and may be made up of fine-grained sandstone. The size of the marbles varies, but they typically range from a few centimeters to several inches in diameter.
The formation of these marbles is attributed to a combination of chemical weathering, mineral precipitation, and the action of wind and water over time. Moqui marbles are thought to have originated around 150 million years ago during the Jurassic period, when the region was a vast coastal plain subject to fluctuating groundwater levels.
In addition to their geological significance, moqui marbles have cultural importance among Native American tribes, who historically associated them with spiritual or healing properties, which is how they earned the nickname “shaman stones.”