DODGE COUNTY: Opal has been found as a microscopic component of the oolites in the Mayville iron ore (Hawley and Beavan, 1934).
FLORENCE COUNTY: Colorless hyalite opal is found as encrustation on feldspar in the zoned, lithium-rich pegmatites in sec. 22 T.31N. R.17E. in Fern Township. This material fluoresces green under short wave ultraviolet light (Koehler, 1988).
JACKSON COUNTY: Common opal is the major component of a bed of diatomaceous earth in a bed 15 centimeters thick deposited in a Pleistocene lake and now outcropping near Hixton, N 1/2 SW SE Sec. 17 T.22N R.5W (Andrews, 1966).
MARATHON COUNTY: Hyalite opal encrustations are common in the pegmatites of the Wausau pluton, such as are exposed in the rotten granite quarries south of Rib Mt. Sec. 19 and 20 T.28N R.7E.
VILAS COUNTY: Bottom mud in a number of county lakes contains diatomaceous earth sediments. Some lakes containing diatomaceous earth are Alder, Allequash, Big Arbor Vitae, Found, Little Crooked, Lost Canoe, Mann, Sweeney and Wolff. This material is likely wide-spread throughout Wisconsin (USGS, 1976).