What Is Petoskey Stone, and Where Can You Find It?
Petoskey stone is a rock and a fossil, often pebble-shaped, that is composed of a fossilized rugose coral, Hexagonaria percarinata. .Petoskey Stones are also commonly found in Iowa, Indiana, Illinois, Ohio, New York, Canada
Grand Staircase Escalante National Monument - Moqui-Marbles
"The abundant concretions found in the Navajo Sandstone consist of sandstone cemented together by hematite (Fe2O3), and goethite (FeOOH). The iron forming these concretions came from the breakdown of iron-bearing silicate minerals by weathering to form iron oxide coatings on other grains." en.wikipedia.org/wiki/Navajo_Sandstone#Iron_oxide_concret...