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Rough and faceted sapphires from Rock Creek, Montana

Montana sapphires occur at three commercial secondary deposits: Rock Creek, Dry Cottonwood Creek and the Missouri River area. These rough and faceted Rock Creek stones show the range of permanent colors after routine heat treatment.

"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...

Moqui Marbles, Utah Iron Oxide concretions (Moqui Marbles) Moqui Marbles, Hematite Concretions, from the Navajo Sandstone of Southeast Utah. The Navajo Sandstone is also well known among rockhounds for its hundreds of thousands of Iron Oxide concretions