Formula: Al2O3 Hexagonal
Description: Corundum is most famous for its rare colored varieties: the bright red ruby and the deep blue sapphire. More often, it is found as drabber blue to blue-gray crystals. Corundum forms in rocks poor in silica. It can be found in silica-under saturated igneous rocks such as syenite or in certain schist with unusually high amounts of alumina and low amounts of silica. Corundum survives erosion and weathering, and can be found as a heavy mineral in sands and gravels. Corundum is extremely rare in Wisconsin, being found exclusively as a redeposited sedimentary mineral. Even these few reports must be viewed with caution, as red garnet is easily mistaken for ruby and blue zircon mistaken for sapphire.
BARRON COUNTY: Trace amounts of corundum have been found as tiny grains in the Barron quartzite in the Blue Hills east of Rice Lake (Ernst, et al., 1980).
BROWN COUNTY: Eckert (1980) reports corundum as both sapphire and ruby found on rare occasions in the gravel pits of the Green Bay area.
CHIPPEWA COUNTY: Corundum is sparsely found as tiny grains in the Barron quartzite in the Blue Hills east of Rice Lake (Ernst et al., 1980).
DODGE COUNTY: Hawley and Beavan (1934) reports a small number of clastic corundum grains within the Mayville iron ore.
OUTAGAMIE COUNTY: Rare ruby and an occasional sapphire are reported from stream gravels throughout the county, particularly in the gravel pits near Oneida (Eckert, 1980) and Seymour (Crowns, 1976).
RUSK COUNTY: Corundum is sparsely found as tiny grains in the Barron quartzite in the Blue Hills east of Rice Lake (Ernst et al., 1980).