State Rocks, Minerals, and Gems


State mineral: fluorite

Southeastern Illinois once produced the majority of fluorite in the US, and it is the site of the most recently active fluorite mine in the country.


State rock: Salem limestone

Salem limestone is a high-quality building material that may be seen in the facades of buildings from the Empire State Building to the Pentagon to Yankee Stadium. It has also been used in the construction of scores of university and government buildings throughout the country.


State rock: geode

The town of Keokuk in southeastern Iowa is one of the most productive sites of geodes in the world. The formation of these geodes is not completely understood, but it is thought that when the bedrock initially formed, spherical concretions of calcium carbonate were accumulated among the layers of mudstone. Subsequently, water richer in silicon dissolved these concretions, leaving an open space in the rock where silicon, as quartz, could precipitate. In this way, the quartz crystals grew slowly from the outside inward, frequently leaving a hollow space in the center. Because quartz geodes are much harder than mudstone, the hollow orbs may often be found loose on the ground after the bedrock has been eroded away.


State rock: Petoskey stone

Petoskey stone is fossilized Hexogonaria coral from the Devonian period.

State gem: chlorastrolite

Also called Michigan Greenstone, chlorastrolite is a variety of pumpellyite with a distinctive “turtleback” pattern created by its interlocking green crystals. It formed in void spaces in basalt during the Midcontinental Rift event, and, because it is substantially harder than the surrounding igneous rock, it is often found as loose, bean-sized pebbles. It is especially abundant on Michigan’s Isle Royale.


State gem: Lake Superior agate

Characteristically red, Lake Superior agate originates near its namesake lake, but glaciers have redistributed enough of the stone that it may be found throughout much of the state.


State gem: Ohio flint

Ohio flint is a high-quality chert found throughout eastern and central portions of the state. Native Americans used Ohio flint to make tools as early as 12,000 years ago. It is a variety of microcrystalline quartz and a favorite of lapidarists.


State mineral: galena

Galena played an important role in the founding of the state of Wisconsin because early settlers mined it as a lead ore.

State rock: red granite

Red granite is mined extensively in Wisconsin and, depending on its quality, is used to make products from countertops to gravel. It was formed around 1.85 billion years ago when an island arc crashed into the Superior Upland.