Most Moqui Marbles come from the Navajo Sandstone Formation in the state of Utah. In the US, they get their name from the Moqui Indians who inhabited the lands of southern Utah where they are found. They are also referred to as "Thunderballs," "Boji Stones" and "Shaman Stones." They come in a variety of sizes ranging from that of common marbles to that of a baseball. Found during the excavation of ancient ruins around the world, some archeologists believe Moqui Marbles have been used for centuries by Shamans and other mystics. These mostly smooth, elliptical or round balls of compacted sandstone are encased in a "shell" of hematite (an ore of iron). Geologically, they are referred to as "hematite concretions." Geologists believe that they were formed underground 25 million (or more) years ago as minerals precipitated from flowing groundwater. After the marbles formed, the surrounding Navajo sandstone slowly eroded away over millions of years, so the hard, erosion-resistant concretions accumulated on the ground, often in great numbers. they come in 2 shapes, and are either round like a ball (feminine) or more disc-like with a ridge (masculine). Size ranges from 1.25" to 1.75” (3.2-4.4 cm) in diameter