Each bid is for 3 Moqui Marbles from the Navajo Sandstone of Arizona. These iron oxide concretions are important because of the nature of their formation, similarity to Martian Spherules, and the implications of life on other worlds:

"Evidence suggests that microbial metabolism may have contributed to the formation of some of these concretions. These concretions are regarded as terrestrial analogues of the hematite spherules, called alternately Martian 'blueberries' or more technically Martian spherules, which the Opportunity rover found at Meridiani Planum on Mars." (Wikipedia)

The images show typical specimens just like the ones you will receive.

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