Formula: Varies
Description:
Jade refers to a generally green fine-grained translucent rock consisting of either the pyroxene mineral jadeite or an amphibole in the tremolite-ferroactinolite series (the latter termed nephrite jade). Beginning in the late 1950s, attempts were made to promote a fine-grained metavolcanic rock as nephrite jade that was collected near a branch of the Rib River in Marathon County. An amateur collector and lapidarist, Guy Wilson, found the material as loose float blocks and, eventually, small outcrops. G.F. Hanson, state geologist at the time of the discovery, was sent several specimens. Using optical and x-ray diffraction techniques, he verified that the material was indeed mostly intergrown fibers of tremolite-ferroactinolite, hence could be defined as nephrite jade (WGNHS files). There was some debate as to the degree of translucency and color needed to technically qualify as “jade.” Wilson (1958) reports tumbling this material as well as making attractive rings and cabochons from it. He reports that the jewelry shows a “great many delightful patterns.” He described the material as follows: “Although most of it is opaque green, some has a bluish cast. Mottled and veined greens and blacks, greens and whites, some with yellow and yellowish patches, as well as solid greens occur. I have found one small brown piece.” (Wilson, 1958).
In 1971 two prospectors, George Friedrich and A.J. Werner, were reported as finding a nephrite outcrop also in the Rib River area. This was again verified as nephrite by Wisconsin Geological and Natural History Survey geologists, as summarized in the December 1, 1971 issue of The Lapidary Reporter.
More recently a bedrock deposit was found by David Towele in northern Marathon County, again in the Rib River drainage. This is being actively marketed (2015) by a company called Wisconsin Jade. Samples of this material are verified to be primarily fine-grained tremolite-ferroactinolite and thus a nephrite jade. It was also endorsed as nephrite jade by the Gemological Institute of America in 2014. The material appears to be a highly brecciated mafic to ultramafic rock that was subsequently replaced by tremolite-ferroactinolite. It has a wide range of colors ranging from nearly white to dark green, and occurs in large blocks suitable for a variety of decorative uses. The brecciation gives large slabs a variety of interesting patterns and colors.
Some dealers have sold dark green antigorite from an undisclosed locality in Wood County as “Wisconsin jade.” Antigorite is a serpentine polymorph and not true jade. Although it is attractive and can be polished, the material is relatively soft and should be looked upon only as a jade “look-alike.”