Seam in sandstone displaying precious opal, showing characteristic play of colors. From Australia. Field of view is about 7 cm. left to right . (Photo by W. Cordua.)

Formula: SiO2.nH2O Amorphous


Opal is a mineraloid composed of spherical silica aggregates with various amounts of interstitial water. It forms under a wide range of low-temperature geological conditions. Opal may be deposited by silica secreting organisms such as diatoms in lakes (forming a sediment called diatomaceous earth) or by land plants such as bamboo. It is deposited as a result of near-surface hot spring or hydrothermal activity. A clear variety, called hyalite forms in granite pegmatites. Opal may be deposited by the action of silica-rich groundwater in rock pores. Precious opal is a rare variety whose beautiful play-of-colors is the result of light diffracted by microscopic silica spheres have a particular size and spacing. No precious opal is known from Wisconsin, although common opal has been found in several areas.

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.39N. 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. These are thought to be primarily ground water precipitates (Falster et al., 2000).

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).