Occurring in all colors of the rainbow and used for a variety of manufacturing purposes, fluorite is recognized as the state mineral of Illinois. Fluorite crystals formed 150–200 million years ago when hot water containing fluorine and other minerals was forced up through cracks in the earth where it interacted with the calcium-rich limestone bedrock. Crystals formed along cracks and in other open spaces in the rock.

Fluorite (fluorspar) mining got its start in Illinois in 1842. The mineral is usually referred to as fluorite, while the product that is mined is called fluorspar. It was used as a flux to help remove impurities while smelting metals like iron and aluminum. It also is used in products ranging from optical lenses to fluoride (derived from fluorite) in toothpaste. Fluorspar production peaked in the 1960s when an average of 118,820 tons was mined annually. Fluorspar mining eventually became unprofitable due to competition from overseas producers, coupled with the high costs of underground mining. The last mine in Illinois closed in 1995. Fluorspar is no longer mined in the United States.

Examples of fluorite crystals in a variety of colors are on display in the Illinois State Museum’s Changes Exhibit. Pure fluorite (composed of fluorine and calcium) is colorless, and any colors visible are due to other minerals being present at the time the crystals formed. Fluorite specimens are highly prized by collectors and museums.