Formed by precipitation from ground and surface waters, calcite is abundant throughout Wisconsin. Among the many varieties found in the state are those resident in the zinc and lead mines of Iowa County and the stunning crystal specimens discovered in mines of the Gogebic Iron Range. Some of the white calcites of Rusk County are fluourescent under ultraviolet light. The crystal calcite found in quarries near the Wisconsin Rapids fluouresces red.
Lake Superior Agate
Beautiful agates are relatively common along the Lake Superior shore. According to Tom Evans, assistant director of the Wisconsin Geological Survey, the hard agates washed out of softer basaltic lavas that cropped out of the shoreline after many years of wave erosion. An igneous rock stained by the presence of iron, agates are typically banded in red, yellow and orange.
Bright green malachite ranks among the most sought-after minerals in Wisconsin. Specimens can be found in a variety of remarkable shapes -- some globular, some as fibers, still others as coils. Malachite is often confused (and misidentified) with other green minerals. True malachite will effervesce in hydrochloric acid. As the mineral is commonly the result of the weathering of copper ores, it can be found in copper mines across much of the state.
The most abundant mineral in Wisconsin, quartz is found practically everywhere in the state and in a wide array of forms. Quartz crystals several inches long have been collected from Williams Quarry in Columbia County. Other specimens occur as Lake Superior agate and can be found on the Apostle Islands. Jackson County is home to a mottled, reddish-brown variety called "zebra sandstone." Grey, pink, green and milky crystals run in long veins along the Necedah Bluff, in Juneau County.