From Ohio History Central
Minerals are naturally-occurring elements or compounds that are the constituents of igneous, metamorphic, and sedimentary rock. In unusual circumstances, minerals may grow into typical crystal forms that make them desirable to collectors.
Most people would not think of Ohio as a state that would yield a surprising variety of mineral specimens; however, there are many Ohio collectors who have assembled collections of museum-quality specimens. Beautiful and colorful crystals of such minerals as calcite, celestite, pyrite, selenite gypsum, and sphalerite, among others, can be found along with small flakes of gold, colorful flint, and rare meteorites and diamonds.
Minerals occur in a variety of areas in Ohio in association with sediments or sedimentary rocks. Many of them have crystallized from fluids that flowed through the rocks and deposited minerals in open spaces. A well-known mineral district in the state is in northwestern Ohio (Findlay Arch mineral district) where sometimes-spectacular crystals of calcite, celestite, dolomite, fluorite, and others are present in small to large vugs—or cavities inside rocks—in dolomites of Silurian age. Unfortunately, natural outcrops of these rocks are rare in this glaciated area and most exposures are in quarries that dot the area. Many of these quarries are no longer open to the public for collecting. A small area in southern Ohio, in parts of Adams and Highland counties, is called the Serpent Mound zinc district. It derives its name from the presence of the mineral sphalerite, which is a major ore of zinc. However, it is not known to be in economic quantities in this area. Other minerals, including calcite and barite, occur in this district along with geodes. Rocks of Pennsylvanian age in eastern Ohio are known for hematite nodules, petrified wood, and ironstone septarian concretions. Pyrite and marcasite crystals occur in association with the Olentangy Shale and Ohio Shale in the outcrops of these units in southern, central, and northern Ohio.
Detailed information on the occurrence and collecting of minerals in Ohio is given in the book Minerals of Ohio, published by the Division of Geological Survey. Collectors should always ask permission from the landowner before collecting fossils, minerals, or other specimens. Many spectacular specimens of Ohio minerals are on display at the Ohio History Connection and at other natural history museums in the state.
Calcite – Calcium carbonate. Calcite is common as vein fillings in many rocks in western and central Ohio. Silurian dolomites in northwestern Ohio yield clusters of large crystals ranging from clear to dark brown. Many have a golden color.
Celestite – Strontium sulfate. Northwestern Ohio produces crystals of this mineral in colors ranging from white to a beautiful pale blue. Crystal cave on South Bass Island is a large vug filled with very large crystals of celestite.
Diamonds – Carbon. At least six diamonds have been found in Ohio in sediments deposited by glaciers of the Pleistocene Ice Age. All were small crystals. As with gold, these minerals were carried to Ohio from Canada by glaciers. In recent years, diamond-bearing kimberlite pipes have been discovered in the Canadian shield and they may have been the source for rare Ohio diamonds.
Dolomite – Calcium magnesium carbonate. Interesting crystals, ranging in color from white to brown to pink, occur in northwestern Ohio in association with other minerals.
Flint – Silicon dioxide. Flint is Ohio’s official gemstone. Amorphous silica that occurs in nodules or beds in Devonian limestones andparticularly in Pennsylvanian limestones. It is thought that the silica was derived from the siliceous spicules of sponges. The most conspicuous bed of flint is associated with the Pennsylvanian-age Vanport limestone at Flint Ridge in Licking and Muskingum Counties. Although most flint is gray or black in color, Flint Ridge flint is characterized by its light color with hues of red, green, yellow, and other colors. Flint was quarried by Native American cultures for spear points, knives, scrappers, and other functional and ceremonial objects. Today, collectors polish Ohio flint into colorful jewelry. The aboriginal quarries and displays of Flint Ridge flint are visible at Flint Ridge State Memorial.
Fluorite – Calcium fluoride. The Findlay Arch mineral district produces interesting crystals of fluorite in brown and sometimes purple, green, and yellow, usually in association with calcite and other minerals common to the district.
Galena – Lead sulfide. This gray-colored heavy mineral is an ore of lead. Small cubic crystals of galena occur in the Findlay Arch mineral district.
Gold – This native element occurs as small flakes and is found by panning gravel in the beds of modern streams. Gold was transported to Ohio by glaciers of the Pleistocene Ice Age and modern streams have eroded the glacial sediments and concentrated the gold particles. Many streams in the glaciated portion of the state, or rivers draining the glaciated areas have produced small amounts of gold.
Gypsum – Hydrous calcium sulfate. Gypsum occurs in massive form in northern Ohio, where it was mined until recently. Moderately large, well-formed crystals of clear selenite gypsum are found in northeastern Ohio in silts and clays deposited in beds of former glacial lakes.
Hematite – Iron oxide. This mineral occurs as reddish to grayish nodules in rocks of Pennsylvanian age in eastern Ohio. These nodules are comparatively heavy because of the iron content. Early charcoal-fired iron furnaces in Ohio used hematite ores as the source of iron.
Marcasite – Iron sulfide. Gold-colored marcasite look similar to pyrite but has a different crystal structure. It is found primarily in the Devonian-age Ohio Shale.
Melanterite – Hydrous iron sulfate. Melanterite was formerly known as copperas. This is one of several minerals found in Ohio that are termed efflorescences. They occur as small, delicate crystals on the surface of coal beds and shales such as the Ohio Shale where evaporation draws mineral-bearing waters to the surface.
Pyrite – Iron sulfide. Pyrite occurs in brassy crystals or sometimes as nodules in Devonian and Pennsylvanian shales. This mineral is commonly known as “fool’s gold” because it has been mistaken so often for gold due to its gold color.
Quartz – Silicon dioxide. In addition to flint, quartz occurs as small crystals in association with dolomites in western Ohio. The flint beds at Flint Ridge commonly have small vugs lined with quartz crystals.
Sphalerite – Zinc sulfide. Generally found as small, brown to reddish-brown crystals in Silurian dolomites in southern Ohio in the Serpent Mound zinc district. This mineral is also found in concretions in the Devonian-age Ohio Shale.
Strontianite – Strontium carbonate. Strontianite occurs as small white crystals or powdery masses in cavities or vugs in Silurian dolomites in the Findlay Arch mineral district.