The best places to find geodes in Illinois are Warsaw, Hamilton, and Nauvoo, which are areas in western Illinois that lie adjacent to Iowa and northeastern Missouri.
Geodes in this region are known as Keokuk geodes, named after Keokuk town in Iowa, a popular collection place. Keokuk is one of the most famous places where geodes are found worldwide.
However, they also occur in eastern Illinois and other parts of the state but are less common.
What are geodes?
Geodes are hollow or partially filled, nearly globular or egg-shaped discrete rock structures often lined with mineral masses and sometimes crystals. However, they can have other shapes, including oblong, discoid shapes, etc.
Their sizes range from pea-size to basketball or larger. However, most geodes in Illinois are about 2-5 inches. Smaller or larger ones also exist.
Usually, they occur in stratified volcanic rocks like lava flows, ash, and tuff or sedimentary rocks, mainly limestone, dolomite, and sometimes shale and other rocks.
These rocky structures have a rough and bumpy greyish weathering-resistant chalcedony rind on the outside. Thus, they will remain intact as the host rock weathers.
They can have various mineral masses and crystals, especially the colorful agate, chalcedony, opal, quartz, etc. when opened. Also, some can have calcite, amethysts, and other exotic minerals.
Lastly, their mineral composition is different from that of the enclosing rocks.
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Common minerals in geodes in Illinois
The common minerals in Illinois geodes are blue-grey chalcedony, clear quartz, calcite, dolomite, gypsum, marcasite/pyrite, aragonite, ankerite, and goethite/limonite. Pyrite may form filament-like masses.
Other minerals are highly ordered kaolinite, celestite, malachite, smithsonite, wurtzite, hematite, tenorite, selenite, jarosite, and stilpnosiderite.
Lesser common include millerite that forms a fine, hair-like dark bronze mass, with some having petroleum enough to squirt when you open them. However, most of those north of Nauvoo don’t have enough of this oil.
Lastly, some of the minerals in these geodes are drusy, i.e., they have well-formed mineral crystals that project from the surface inward to the hollow center.
In which rocks do Illinois geodes occur?
Geodes in Illinois occur in sedimentary rocks, i.e., shales, shaley dolomites, and limestone, in the lower Warsaw Formation or rock unit.
In this formation, they appear in certain beds and are not randomly scattered.
The lower Warsaw Formation hosts Mississippian age rock units widespread in the tri-state region of Iowa, Western Illinois, and northeastern Missouri.
How did geodes in Illinois?
Geodes found in the lower Warsaw Formation didn’t exist during the accumulation, early sedimentation, or mud compaction that formed the lower Warsaw Formation about 340 million years ago.
Instead, these geodes formed from nodules and calcite concretions present in this formation. Concretions are compact spherical or disc-shaped mineral masses that occur in rock beds. They are common in shale, limestone, or sandstones.
On the other hand, nodules are bumpy, irregularly to nearly rounded lumps, masses, or mineral aggregate in sedimentary rocks.
Mineral-rich groundwater dissolved these concretions and replaced the calcite with quartz and other minerals.
For instance, factors like water acidity and a change in chemical composition caused by the groundwater could have resulted in chalcedony replacing calcite, forming the outer, weathering-resistant rind.
As more mineral-rich fluids seeped into the concretion, they dissolved calcite inside, partially replacing the formed cavity with quartz and other mineral masses and crystals.
Dissolution and crystallization or precipitation of minerals may co-occur, i.e., syngenetic.
One evidence supporting the origin of geodes in Illinois is the calcite concretions in this formation. These concretions resemble geodes in size, shape, and space location.
Lastly, these calcite concretions may have formed from plant and animal remains, i.e., organic matter accumulation. The decomposition of organic matter without oxygen (anaerobic) created an alkaline condition that favored calcite to precipitate, forming concretions.
Geode hunting in Illinois
Geodes in Illinois occur in stream beds, erosion gullies, or dug from outcrops, especially on the exposed lower Warsaw formation. Some can occur among gravel or drift deposits or on cut roads.
Note: Some areas fall on private properties if you are going for rock-hounding or geode hunting. Ensure you first inquire.
Some areas to hunt for geodes include:
1. Hamilton area
Hamilton area in Hancock County, west-central Illinois, along the Mississippi River is part of the Keokuk geode region. It has many places that include:
- Areas with gravel and glacial drift deposits have geodes with blue-grey chalcedony
- Crystal lined South of Keokuk bridge on banks of streams and gravel
- Hamilton quarry geodes lined with marcasite, pyrite, sphalerite, and calcite rhombs
- Extensive exposure to geodes in the South of Crystal Glen Creek. Here, they are lined with any combination of aragonite, malachite, pyrite, jarosite, calcite, chalcopyrite, geothite, pink dolomite, quartz, pyrolusite, selenite, sphalerite, marcasite, smithsonite and barite.
2. Niota area
Niota area in Hancock County, Illinois, has many locations with geodes that often contain hydrocarbons.
Geodes in the Niota area occur in stream banks, gravels, and excavated areas. Locations include
- Tyson Creek, about two miles southern of Niota
- Dewdrop Diamond Locality midway from Niota to Dallas has geodes filled with pink and blue chalcedony.
- Spillman Creek south of Pontoosuc. It is about 4 miles east of Niota on Highway 96 and has geodes with pyrite or calcite on chalcedony or sphalerite that grow on calcite, chalcedony, or quartz.
3. Mississippi River shores and banks in Warsaw formation
Mississippi River shores in Warsaw formation around Nauvoo, Warsaw, and Hamilton, all in Calhoun Country, Western Illinois, have geodes.
They have chalcedony with a botryoidal mineral habit, i.e., a rounded, bumpy surface reminiscent of grapes. However, in rare instances, they can have quartz or amethyst crystals.
4. Dallas City
Dallas City in Hancock country, especially Dallas Creek, has geodes lined by brown calcite, chalcopyrite, and, on rare occasions, malachite.
3. McKee Creek
About two miles from Perry in Brown County, Illinois, McKee Creek has geodes lined by mostly calcite and sphalerite.
Commercial sites
Several commercial sites or private farms allow miners to hunt or collect geodes. Each has its opening hours and prices.
You must carry your tools, including a bucket, in most of these sites. Also, inquire about pet rules and any other restrictions beforehand.
Some of these commercial sites in Illinois include:
1. Jacob’s Geode Farm in Hamilton, Hancock County, Illinois, USA
This farm is a private property belonging to Gary Jacob and has a quarry on the Warsaw Formation. You can pay to dig or hunt for geodes.
Most specimens have highly ordered kaolinite. However, as you dig for these hidden treasures, have fun, but be mindful of your neighbors.
2. Dennis Stevenson Geodes
This private commercial site is at 625 S. 18th Street, Hamilton, Illinois. You can collect and pay $20 per bucket of geodes you collect or $15 each if you take two.
3. Nick’s Geodes
This farm is at 251 North 7th Street, between Nauvoo in Illinois and Keokuk, Iowa. You will pay about $20 per 5-gallon bucket of geodes.
Other gemstone in Illinois
Illinois has gemstones like agate, chalcedony, fluorite, pyrites, marcasite, jasper, chalcopyrite, calcite, smithsonite, and malachite.
Others are sphalerite, siderite, galena, aragonite, barite, cerussite, dolomite, goethite, bornite, quartz, selenite, and rare diamonds.