The Wondrous Geode
Based on a temporary exhibit shown in 2000
by Alan Goldstein
Falls of the Ohio State Park
What is a geode?
A geode is a spherical rock lined with a rim of quartz which can be hollow or solid and may contain other minerals inside. Indiana and Kentucky geodes often contain an external pattern resembling cauliflower.
Geode broken in equal halves. The left contains a large calcite crystal in addition to quartz.
Meade Co., Kentucky
Exteriors of two typical weathered geodes
Where are geodes found in the central United States?
Geodes are most commonly associated with Mississippian-age rock layers. With less frequency, they are found in other time periods such as Ordovician and Devonian. The best rock exposures stretch from east central Tennessee to central Kentucky and south-central Indiana, as well as northeastern Missouri and corresponding layers in northwestern Illinois and eastern Iowa.
What is the difference between a geode, a nodule or concretion and a vug?
A nodule is generally solid spheroidal in shape and may be composed of quartz, ironstone, pyrite or other minerals.
Flint nodule, Breckinridge Co., Kentucky
A concretion is a nodule with concentric banding as a result of trace amounts of iron, clay or other impurities.
Chert concretion with a bull's-eye pattern Ironstone concretion
Floyd Co., Indiana Jefferson Co., Kentucky
Vugs are crystal pockets without the quartz lining. In the Midwest U.S., they are most common in limestone, but can occur inside fossils in shale deposits.
Vug (4 cm wide) lined with orange dolomite with a large blue barite crystal
Bath County, Kentucky
Vug (15 cm wide) lined with pink dolomite filled
with a large calcite crystal.
Bath County, Kentucky
Calcite in sharp crystals. This is a vug, there is no quartz rind.
Taylor Co., Kentucky
Silurian Uncinulus brachiopod with calcite
Clark Co., Indiana
How did geodes form in Indiana and Kentucky?
A commonly accepted hypothesis is the process that creates a geode starts as a void within a fossil or gypsum nodule buried within / surrounded by sedimentary rock. Gypsum nodules are created in sediments associated with extremely salty water. This mineral can readily dissolve and precipitate depending on conditions. Dissolved gypsum migrates from its source and is thought to precipitate in small voids in underlying sedimentary rock when conditions are favorable. As the nodule grows, it displaces the surrounding sediment slightly.
Eventually ground water containing dissolved silica deposits a rim between the gypsum and the surrounding sedimentary rock. As the amount of silica (quartz) increases, it pushes the sedimentary rock further and further outward forming the exterior cauliflower texture.
When the buried the gypsum nodule begins to dissolve, it opens space so that quartz (or less commonly other minerals) can accumulate on the exterior and begin to grow inward. When the gypsum has completely been removed, larger crystals of quartz, calcite and other minerals will grow crystals inside.
Proto-geode with gypsum, Hardin Co., Kentucky
(Indiana Geological survey photo)
How do fossils become geodes?
Animals that are buried quickly may contain an empty void. These hollows may fill in with gypsum by ground water. The growth of gypsum will push the fossil outward against the sedimentary rock. As it slowly expands, the calcium shell material may be dissolved away or become fragmented - separated by silica. The more expanded the fossil becomes, the less it resembles the original fossil. Eventually, the fossil may disappear leaving an ordinary geode.
Snails (Geodes) Nautiloid Cephalopod (Geode)
Lawrence Co., Indiana Bullitt Co., Kentucky
Horn Corals (Geode) Brachiopod (Geode)
Clark Co., Indiana Clark Co., Indiana
Early geodized crinoid columns An "exploding" crinoid stem - early geode
Hardin Co., Kentucky
An oddity - a horn coral attached to a geode
Why are most geodes solid?
The wall of the geode will thicken as long as there is water carrying dissolved minerals that percolate through the surrounding bedrock. Some geode locations are dominated by those with very thick walls – in some instances over 95% of the geodes may be solid. When thin-walled geodes are exposed by natural erosion, they often disintegrate with the surrounding rock. Gullies and washes are the best spots for naturally occurring thin-walled geodes. Those that are found may have loose crystals (rattlestones) or may be filled with mud. Eroded geodes with thin-walls rarely for attractive specimens because clays usually coat the crystals. Quarries and road cuts that expose fresh rock are the ideal location to find thin-walled geodes.
Some geodes contain very little quartz, the wall and crystals are dominated by calcite. A close examination will reveal a very thin rind of quartz (chalcedony). If there is an exterior of calcite, then this is a vug, not a geode – even if the overall shape is the same.
Thin-walled Geode Thick-walled Geode
Quartz rim (white), brown calcite inside Chalcedony and calcite inside
Why do geodes in streams typically contain only quartz?
The thick quartz rind in geodes is very resistant to the effects of erosion and weathering – but it is not impermeable to water. Once a geode is eroded from surrounding bedrock, it often ends up in a creek where microscopic cracks allow water to percolate inside. Many minerals are water soluble and this tends to dissolve or etch the crystals, leaving "indestructible" quartz and an occasionally a residue of other minerals behind. The delicate crystals observed in many geodes in this exhibit are collected from fresh exposures.
Geodes in a stream, Boyle Co., Kentucky
Explore the variety of minerals found in geodes of this area (the main exhibit gallery!)
"Exhibit" of Minerals in Geodes
How small do crystals get? (Microminerals are included in the album above.)
Crystals start small and grow larger over time. However, some elements or compounds may never occur in sufficient amounts to form large crystals. Crystal growth is complex and the chemistry of groundwater may change, allowing minerals to crystallize and then begin to dissolve away or change their form.
Some minerals tend to only form in very small crystals. These are called microminerals. While these minerals require magnification to be observed or studied, the size of crystals may range from a few microns to millimeters. The tiniest crystals are best studied with a scanning electron microscope.
These are photographs of some of the fascinating microminerals and microcrystals that have been found associated with geodes presented in this exhibit.
Some crystals are small enough to be appreciated with low power magnification, but too large for an electron microscope. The aesthetics of some of them are a natural form of art and sculpture.
How does one open a geode?
Carefully – very carefully! Geodes are notoriously difficult to open ‘just right.’ One can heft them or do density tests to determine how thick the wall is, but when push comes to shove, getting the perfect geode is really a result of sheer luck!
Safety gear and keeping a distance from anything or anyone you love is paramount. If you use a hammer and chisel, remember that quartz is glass – and a flying shard can be just as dangerous! We won’t go into detail about opening one other to say that – unless you have a diamond saw to score the perimeter or a peripheral crack already exists, expect the geode to break imperfectly. You can also damage or obliterate other minerals within that are invariably softer than quartz.
Created January 13, 2012, Updated May 27.