Wisconsin Rocks and Minerals

The Badger State is home to a wide variety of rocks and minerals, some of which you may find familiar and others you may not recognize. In this interactive guide, the Wisconsin Geological and Natural History Survey takes you on a journey through the state, exploring our most important rocks and minerals.

Granite is a hard, coarse-grained igneous rock that most often contains the minerals quartz and feldspar. Ranging from red, to pink, to light gray in color, granite is commonly found in the north-central part of Wisconsin.

Granite is among the oldest of any rocks found in the state, being up to 2.7 billion years old—from the Precambrian age. It finds a variety of uses in construction and industry today. The coarse grain of granite and its visually appealing mineral composition makes it a popular choice for kitchen countertops. 

Dolomite is a fine- to medium-grained sedimentary rock that forms when limestone encounters magnesium. Limestone is made when the calcium carbonate skeletal materials of sea creatures are deposited on the seafloor. Consequently, marine fossils are commonly found in dolomite. 

Two major transgressions and regressions of tropical seas during the Cambrian, Ordovician, and Silurian Periods—over 400 million years ago—deposited dolomite across parts of Wisconsin. 

Sandstone is a sedimentary rock that forms from compacted grains of sand. It is a fine to medium-grained rock composed mostly of quartz. Sandstone ranges in color and is often white, yellow, gray, brown, or even red.

Most sandstone in Wisconsin is about 500 million years old, but red sandstone is found to be 1 billion years old—from the Precambrian. 

Sandstone has multiple industrial purposes. Glass manufacturers utilize sand as a primary ingredient, melting it down at extreme heat. Fine-grained Wisconsin silica sand is prized for its congruent particle size, making it ideal for hydraulic fracking, a method of extracting oil and gas from the Earth. 

Shale is a fine-grained sedimentary rock formed of compacted grains of silt, mud, or clay. It is a softer rock—only a 3 on Mohs Hardness Scale—and can be black, gray, green, brown, or red in color. A unique characteristic of this rock is the thin layers it is composed of.

Shale in Wisconsin can range from 400 million to 1 billion years old, and it was deposited during the Precambrian through the Devonian Period. Since shale is deposited in gentle, slow-moving water, it was likely deposited in Wisconsin near the peak of the Sauk and Tippecanoe sedimentary sequences.

Shale is known as a source of fuel if it is an “oil shale.” Oil shale contains kerogen, which releases petroleum and natural gas when heated. While mature oil shale is not found in Wisconsin, it can be sourced from upper Devonian deposits in neighboring Illinois. Additionally, shale is an ingredient used in creating brick, tile, and cement when it is ground into a powder.

"Gravel" is a term referring to any loose collection of small rocks. These rocks can be of any origin. Much of Wisconsin’s gravel originated as glacial outwash—an effect of melting glaciers. This gravel is now deposited in broad plains and river valleys across the state.

Gravel is ubiquitous in unpaved roads and driveways. It's also used as an aggregate in concrete, and serves as the foundation for nearly all road construction projects.

Quartzite is a fine-grained metamorphic rock, formed when a quartz-rich sandstone is altered by heat and pressure. The color can be a brownish-red, pink, or gray, and the rock is almost completely composed of silicon dioxide. 

In Wisconsin's Baraboo Range, "Baraboo quartzite" is the defining feature of the landscape. These quartzite deposits—from 1.8 billion years ago (the Paleo-Proterozoic Era)—are noted for their maroon hue. This unique coloration is attributed to the presence of iron and other impurities in the quartzite.

Quartzite is a common choice for countertops due to its lighter appearance and properties similar to granite. It is also used as an abrasive when grinding things up into powders. 

Iron ore is any rock that contains mineable iron. These rocks are metamorphic, fine- to medium-grained, and can range in color from dark gray to rusty red. In Wisconsin, the most important iron-bearing minerals are magnetite, hematite, and goethite/limonite. Magnetite is highly magnetic; hold a strong magnet up to a rock to test whether it contains magnetite.

The majority of Wisconsin's iron ore is found in the far north, near the towns of Mellen and Hurley. These deposits are a part of the Gogebic Range, an iron range that stretches into Michigan's Upper Peninsula. It is about 2 billion years old, from the Precambrian.

Iron ore in Wisconsin is neither as plentiful nor accessible as it is in Minnesota or Upper Michigan. As a result, Wisconsin ended its iron mining operations during the previous century, while Minnesota and Michigan still actively mine for iron in their northern ranges.

Zinc ore is any rock that contains mineable zinc. In Wisconsin, it is a metamorphic medium- to coarse-grained rock commonly found in the southwestern portion of the state. The primary mineral and ore of zinc is sphalerite, which is gray or silver in appearance. Since sphalerite is only a 3.5–4 on Mohs Hardness Scale, it can be scratched with a penny!

In order to extract zinc from zinc ore, sulfur must first be removed through the roasting of concentrates. Then, either electrolysis or smelting is used to extract the zinc. Zinc is commonly used in the creation of brass and batteries.

Basalt is an igneous, fine-grained rock. It is black or dark gray with a green tint to it, and is mainly composed of plagioclase feldspar and pyroxene. 

Basalt was deposited in Wisconsin 1, 1.9, and 2.7 billion years ago, during the Proterozoic and Archean Eons. Basalt was heavily deposited in Wisconsin when the Keweenawan Rift (also known as the Mid-Continental Rift) failed about 1 billion years ago and left behind thick layers of igneous rock including basalt. 

Basalt is often used as a crushed aggregate in construction projects. 

Chert is a fine-grained sedimentary rock mainly composed of quartz. It can be white, gray, yellow, or brown in color. In Wisconsin, it is prominent in mounds capped by chert in the southwestern part of the state that are roughly 425 to 475 million years old. However, chert can be found throughout the state and is often found loosely lying at the surface. This is due to its hard nature (a 7 on Mohs Hardness Scale), which makes it resistant to weathering.

Chert is mainly used to create tools—the most unique one being arrowheads. Indigenous inhabitants of Wisconsin have long used chert arrowheads; the numerous archaeological sites within the state inform us about this. Outside of toolmaking, chert today is used as a component of gravel.

Rhyolite is an igneous, very fine-grained rock composed of quartz and feldspar. It is light tan-to-pink when weathered, and dark grey-to-black when unweathered.

In Wisconsin, it is visible throughout Sauk, Columbia, Marquette, and Green Lake Counties. In Green Lake County, the rhyolite is 1.7 billion years old, from the Precambrian. As with many other rocks, rhyolite can be found as a glacial erratic throughout the state.

Rhyolite's main industrial uses are in construction and landscaping. It is an effective building stone and component of aggregate.

Calcite is a colorless mineral composed mostly of calcium carbonate. It is brittle with a waxy luster, and it is only a 3 on Mohs scale.

Calcite can be found in bluffs along the Mississippi River, Lake Winnebago, and Mineral Point. In terms of the geologic timescale, calcite is fairly young in Wisconsin—less than 485 Million years old.

Calcite is mainly used to create scientific instruments.

A concretion is a compact object of matter formed by precipitation found in sedimentary rocks or loose sediment. They are fine- to medium-grained, and are mainly composed of silica, calcite, dolomite, and iron oxide.

A unique type of concretion is a geode, which has a hollow interior lined with inwardly projecting crystals.

In Wisconsin, concretions can be found in Trempealeau County.

Lead ore is identifiable by its gray color and shiny, metallic luster. It is also very soft, only a 1½ on Mohs scale.

Lead from southwestern Wisconsin helped satisfy the nation's hungry demand for ammunition, weights, paints, and more during the 19th century.

Fossils are common across many parts of Wisconsin. They are the modern remnants of ancient life; fossils ranging from tiny trilobites to massive mammoths have been found in the state.

In Wisconsin, fossils are most visible along the coast of Lake Michigan. They are mostly Paleozoic, from 385 to 485 million years ago.

To learn more about Wisconsin's fossils, check out the Wisconsin Geological and Natural History Survey's " Fossils of Wisconsin " webpages.

For more about the minerals mentioned in this story map, visit our  Minerals of Wisconsin  database.

You can also view and download all of our publications for free, including geologic maps, reports, and more, in the  WGNHS Publications Catalog .