Nephrite jade ventifact from the Precambrian of Wyoming, USA. (public display, Wyoming Geological Survey, Laramie, Wyoming, USA)

Nephrite jade (nephritite) is a crystalline-textured to felted-textured metamorphic rock principally composed of one or more amphibole minerals (tremolite to actinolite, Ca2Mg5Si8O22(OH)2 to Ca2(Mg,Fe)5Si8O22(OH)2).

This gorgeous piece of green nephrite jade has a lustrous polish, the result of natural abrasion polishing by winds. Any rock that has natural wind polish is called a ventifact.

Nephrite jade was discovered in Wyoming in the 1930s, resulting in a "jade rush" that lasted for several decades. Most recovered material is alluvial jade, produced by paleoerosion of jade outcrops. Eroded clasts of jade were transported downstream and subsequently buried with other poorly-sorted sediments. Some Wyoming jade has been collected from in-situ outcrops.

This 218 pound specimen of nephrite jade is a large paleoclast, ultimately derived from Precambrian outcrops in the southern end of the Wind River Range (most Wyoming nephrite jade has a geologic provenance in the Granite Mountains.). The Wind River Range mountains were uplifted in the Late Eocene and eroded, producing much fanglomerate debris, which was buried to form the Ice Point Conglomerate (Upper Eocene). The Ice Point Conglomerate itself was buried by post-Eocene sediments and later re-exposed during the Late Pliocene to Early Pleistocene by downfaulting of the Split Rock Syncline. Nephrite jade clasts from the Ice Point Conglomerate were eroded and surface-exposed to abrading-polishing winds during the Pleistocene and Holocene.

Age: Precambrian (probably Proterozoic)

Locality: unrecorded locality at Crooks Mountain, south of the Sweetwater River & south of the western end of the Granite Mountains, southeastern Fremont County, central Wyoming, USA

Provenance: collected by Ray Morgan & Irene Morgan in the 1940s; donated to the Wyoming Geological Survey in 2000.

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