The story
Probably from Moqui (“Hopi”), although a folk etymology claims that it comes from the near-homophonous Hopi mooki (“dead”).
Etymology from Wiktionary, CC BY-SA 4.0.
Moqui marble
Today it means: Any of a group of iron oxide concretions, shaped like small balls, found in the Navajo Sandstone Formation in Utah. Full definition →