Geodes

Geodes are geological rock formations which occur in sedimentary and certain volcanic rocks. Geodes are essentially rock cavities or vugs with internal crystal formations or concentric banding. The exterior of the most common geodes is generally limestone or a related rock, while the interior contains quartz crystals and/or chalcedony deposits. Other geodes are completely filled with crystal, being solid all the way through. These types of geodes are called nodules.

Geodes can form in any cavity that is buried. These can be gas bubbles in igneous rocks, pockets under tree roots, vesicles in lava after a volcanic eruption, or even in animal burrows. Over time, the external wall of the cavity hardens, and dissolved silicates and/or carbonates are deposited on the inside surface. Over time, this slow feed of mineral constituents from groundwater or hydrothermal solutions allows crystals to form inside the hollow chamber. Then the geode makes its way back to the surface through normal geologic processes.

The size of the crystals, including their form and shade of color, vary-making each geode unique. Some are clear as quartz crystals, and others have rich purple amethyst crystals. Still others can have agate, chalcedony, or jasper crystals. There is no easy way of telling what the inside of a geode holds until it is cut open or broken apart.

Geodes are common in some formations in Indiana, Iowa, Missouri, Kentucky and Utah (in the United States). They also are common in Brazil, Namibia and Mexico. A large geode roughly the size of a minivan was discovered in Put-in-Bay, Ohio in the early 20th century. It is known as Crystal Cave, and tours are possible during the summer. In 1967, Iowa designated the geode as the official state rock, and it has a Geode State Park.

Geodes and geode slices are sometimes dyed with artificial colors. Samples of geodes with unusual colors or highly unlikely formations have usually been synthetically altered.Geodes can have mulitple colors in them ( inside the outer layer ).

Image: © Photographer: Elenathewise | Agency: Dreamstime.com

Oregon's State rock, the "thunderegg," may be the best known gem material from Oregon. Thundereggs were not, as believed by some people, ejected from volcanoes, but formed in very soft and friable volcanic ash beds. Solutions containing silica permeated the cinders until favorable points for chalcedony deposition were achieved. Aggregations of chalcedony were deposited, but before the material could fully solidify the center of the concretion split apart, possibly because of shrinkage, permitting the later introduction of additional materials. The resulting star-shaped centers of chalcedony may be in the form of agate, jasper, or in some cases different varieties of opal. Thundereggs are used in a number of ways. One of the most common uses is to simply saw the thunderegg into two pieces, polish the sawed face of each half, and use it as a display or decorative piece; bookends are also made in this fashion. Also, the thundereggs are sawed into slabs from which calibrated and freeform cabochons are cut. Additionally, at least one firm in the United States is manufacturing gem spheres from thundereggs.

Source: Gemstones, An Overview of Production of Specific U.S. Gemstones: U.S. Bureau of Mines Special Publication 14-95

When I first started this site I advised people to be careful when buying geodes on eBay. That still goes but I've found that with geodes, you never know what you're getting until you crack them open.

At first I thought that the really pretty geodes must all come from Mexico or some other country. That the geodes from our mid-west were a waste of time. Not True!

While it is true that most of our geodes from the midwest appear the same when first opened they can be polished up very nicely, and once in a while you run across a few that are really nice.

Some of those really "beautiful" geodes that come up for auction are dyed to enhance their apperance and if that's what you are looking for, that's fine. For most geode buffs, the natural beauty is what we are searching for so if you think that a geode that you are bidding on might be dyed, ask. Then bid accordingly!

I just opened a black geode from Kentucky, hoping to find some different colored minerals inside. To my surpise, the geode was filled with a black tar-like substance.

This is a first for me but some of you "long time" geode hunters have probably seen it before.

If so, how about leaving a comment below and fill me in on what it is, how common they are and does this mean that I don't want anymore black geodes?

Here's a few pictures of it. Sorry that they aren't bigger.

As the name implies, Keokuk Geodes are found in an approx. 35-mile radius of Keokuk, Iowa. There have twenty different minerals found in Keokuk geodes with quartz being the most common.

Iowa General Assembly named geodes as the official "State Rock" in 1967.

Click here to learn more about the Keokuk geodes!

Roughly 32,000 to 14,000 thousand years ago, a large body of water known as Lake Bonneville covered most of western Utah. The lake's wave activity eroded the geode-bearing rhyolite and redeposited the geodes several miles away in the Dugway geode bed area as lake sediments.

Most geodes are typically hollow whereas others are completely filled with massive, banded quartz. The most common mineral found within the geodes is quartz in various colors: clear (rock crystal), purple (amethyst), and pink (rose).

More on Utah geode beds!

Geodes, a term derived from a Greek word meaning earth-shaped, are irregular, roughly spherical bodies.
They can be oblong or shaped like invertebrate fossils (e.g. crinoid calyx). Some are hollow and
lined with beautiful layers and clusters of various
mineral crystals, but others are completely filled by inward-growing crystals. Hollow geodes, relatively lightweight compared with those completely filled, are more desirable because they generally contain a greater variety of minerals that have grown well-formed crystals. Some of Illinois' most beautiful and unusual mineral specimens can be found in the crystal linings of geodes.
Where we find geodes: Geodes found in Illinois range from less than 1 inch to more than 2 feet in diameter, but 3 to 5 inches is the average. They generally occur in limestone, a calcium carbonate (CaCO3), or in dolomite, a calciummagnesium carbonate (CaMg(CO3)2). Although geodes can be found in carbonate-rich rocks throughout the state, one of the most famous geode collecting areas in the country is in a region of western Illinois and adjacent parts of Iowa and Missouri. The region encompasses about a 70-mile radius from the towns of Warsaw, Hamilton, and Nauvoo.
What's in a geode?
A typical geode from western Illinois has an outer shell of chalcedony, a type of cryptocrystalline
quartz composed of silicon dioxide (SiO2). Once the outer shell forms, mineral-rich water still inside the shell may cause more quartz to be deposited and other minerals to form toward the center. Chalcedony, much harder than the host rock of
limestone, helps to preserve the specimen during weathering. As the weaker host rock is eroded, the geodes "weather out" and remain behind. They generally are easy to see because of their shape and the texture of their outer shell. The micro-environment inside the shell is an excellent place for crystal growth. Temperature and pressure changes, as well as evaporation, cause the mineral matter to precipitate. More solutions rich in minerals may seep into the geode later, adding to the quartz crystals or forming other minerals. In addition to the chalcedony of the outer shell, the insides of some geodes are lined with a pronounced bumpy, mammillary form of blue-gray chalcedony. Some specimens also have excellent clear quartz crystals. Ankerite, aragonite, calcite, dolomite, goethite/limonite, gypsum, and marcasite/pyrite are the other minerals most commonly found.

Image: © Photographer: Billbear2000 | Agency: Dreamstime.com

A few people told me that the rock cuts that are made when they put in a new highway or widen an old one are good places to hunt for geodes. Well they had made a four-lane out of the highway going through here a couple of years ago, so off I went.

All that I can say is that this must not be a very good area for geodes because after searching about a dozen rock cuts, I came home empty handed. I never even found any brightly colored rocks!

I did pick up a few rocks that didn't look like any of the geodes that I've seen, but felt pretty light.
When I did my thing with the hammer, they crumbled like eggshells. While they were hollow, they didn't offer up anything that even looked like quartz or any other crystals. Just some dirty looking dust.

I did find a few pieces that appear to be petrified bone, but I'm not even sure of that.

Maybe I'll go back and look again, but I'll probably be wasting my time.

The main thing anyone needs to know about buying geodes on eBay is to "be sure that you know what you are getting".

Some geodes have been dyed or altered. If you're just looking for an unusual decoration, this might suit you just fine. If you're an experianced collector you probably already know what to look for, but if you're just getting started and want authentic geodes, stay away from geodes with unusual colors or highly unlikely formations.

Ask the seller questions before bidding. Where was the geode found? Can you guarantee it's authenticity? Has it been enhanced?

The inside of a geode can be full of crystals and beautiful, but it can also consist of nothing more than little bb size stones without much color like the one pictured here. So, if you buy uncut geodes be aware that you might not see what you expect when you open them up.

The same goes for "thunder eggs"! Some of them are filled with beautiful crystals, while others can be very mundane and common.

It is sometimes fun to buy a group of thunder eggs on eBay just to open them and see what you have, but I wouldn't advise paying to high a price for them.

Then again, some sellers call nodules or pieces of quartz, geodes. These are not true geodes. Check out the discription of geodes above.

If you're unsure or the seller seems evasive, you're probably better off waiting to bid on a different geode.

Image: © Photographer: Aneese | Agency: Dreamstime.com

A lithophysa (plural lithophysae) is a small cavity found in felsic volcanic rocks believed to be caused by expanding gases in tuffs before solidification. If it has become lined with crystals it may be referred to as a geode or a thunderegg. The word lithophysae is from the Latin for rock bubble. The term vug is also used for similar cavities although vug is usually restricted to mean cavities in rocks formed by the removal of material such as soluble minerals.

It can be tough to find a spot to go rock hunting.

Around here we have one big outfit that owns nine-tenths of the quarries. I contacted them to try to get permission to hunt geodes in one or two of their quarries after hours or on Sundays.

They won't even consider it! I offered to sign a release form so they would not be liable if I should get hurt or anything, but they wouldn't budge.

The state parks don't want you carrying anything out, and most of the land owners are afraid that you'll find something that's worth money. They'd rather leave it there forever rather than let someone else find it.

I'd be interested in hearing some of your stories about finding places to hunt. How about emailing me your stories! jbaguss@yahoo.com Maybe I can post some of them here or do another lens just on rockhounding stories. Put "Rockhound Stories" in the subject line.

Thought that I would add a few pictures of some of my recently aquired geodes.

These are not all that "colorful" but still interesting and you just never know what you've got until you've opened them.

If you have pictures of your geodes, I'd appreciate your comments and picture, letting me know what part of the country they came from. I don't want your "secret" locations! Just a general area such as, southeast arkansas.

I'd like to get an idea as to what type of geode comes from what area.

Sometimes it's nice to let people know what you are doing when you're in the field looking for geodes. It's especially important that you identify yourself and get permission to hunt on private property.

Wearing shirts or hats that say you are a geode hunter or rockhound seems to put people at ease, and for some reason, more apt to give you permission to hunt. It's almost as if they feel that when you are wearing something that states that you hunt geodes or rocks, it must be true.

There are not a lot of items out there that are designed for us so since starting this lens I've done a few different designs for t-shirts, buttons and other gift ideas for the Geode Hunter.

Stop by and check them out. you'll be pleasantly surprised with the cooperation you get when others see you wearing these Geode Hunter and Rockhound items.

I'll be adding more designs as I think them up so stop by often to check them out. Then too, if you have a design of your own, let me know and I can add your design to items in my shop.

Maybe you have a group that would like group t-shirts or hats. There'll be no extra charge for doing your design.

Occasionally, dark bronze,fine, hair-like masses are found inside; these may be millerite (NiS) or a filament-like form of pyrite.

Perhaps the most fascinating geodes are those that contain petroleum, which may be under enough pressure to squirt out when the geode is broken. The enclosing rock north of Nauvoo, where these unusual geodes are found, no longer contains any significant oil. So what is the source of oil in these geodes? What is the origin of the other minerals? We don't know for sure. Perhaps trace amounts of some of the elements that make up the rarer minerals were present in shale layers associated with the carbonate strata. As a matter of fact, the most prolific zone for collecting geodes in western Illinois is in the lower part of the Warsaw Shale of the Valmeyeran Series (middle series of the Mississippian System). These sedimentary strata were deposited in shallow seas that covered what is now the midcontinent about 350 million years ago.

How geodes form:
Geologists have proposed several theories to explain the conditions and processes that form
geodes, but none seems to be entirely adequate to explain all geode features. In discussing the origin of the western Illinois geodes, Hayes (1964) noted that any theory proposed must explain
why the geodes are:
1. essentially confined to a specific stratigraphic interval, the lower part of the Warsaw Shale;
2. usually associated with particular lithologies (clayey, shaley dolomite, and dolomitic mudstone);
3. located in specific zones or beds rather than scattered randomly;
4. fairly uniform in size in a particular zone and round, at least initially;
5. enveloped by laminations in the bedrock that exhibit some thinning of layers above and
below the specimen.

As limey sediments accumulated in shallow midcontinental seas, rounded cavities that are characteristic of geodes could not have existed at the interface or contact of water and sediments. Nor
could they have existed during the earliest stages of sediment compaction and cementation. Therefore,
some feature of a different texture than the host limestone had to be present. This feature either caused geodes to form or was transformed into a geode. Hayes hypothesized that the only features in the rocks that shared enough characteristics with geodes to serve as precursors were calcite concretions (small zones in the original sediment strongly cemented by calcite). The size and shape of these concretions, their position in the

limestone, and their relation to the surrounding rocks are strikingly similar to those of geodes. In several exposures in the region, rock samples may
be found that display all stages of the transition from concretion to geode. Hayes suggested that calcite concretions formed where organic materials (remains of the living tissues of plants or animals)
accumulated with carbonate-rich sediments under quiet-water conditions. The organic matter
decomposed, causing an oxygen-poor (anaerobic), alkaline environment (pH >7) to develop in the
sediments. These conditions encouraged calcite to precipitate from solutions in sediments.

The formation of many features seen in geodes may involve a step-by-step replacement of these concretions by quartz and other minerals. Changes in the chemical composition and acidity (pH) of water in the sediments caused chalcedony to replace the calcite at the outer margins of the concretions.
This process caused the formation of a calcite-concretion core surrounded by a hard, but slightly permeable, shell of chalcedony. Further changes in the composition and pH of the water percolating slowly through the sediment caused the core concretion inside the geode eventually to dissolve,
leaving a hard, hollow cavity in which more chalcedony, quartz, or other minerals could precipitate.

ILLINOIS STATE GEOLOGICAL SURVEY
615 East Peabody Drive
Champaign, IL 61820-6964
217/333-4747 FAX 217/244-7004
Released by authority of the State of Illinois 2004
References
Cote, W.E., D.L. Reinertsen, G.W. Wilson, and M.M. Killey,
1970, Hamilton-Warsaw Area, Hancock County: Illinois
State Geological Survey, Geological Science Field Trip
Guide Leaflet 1970E and 1971B, 22 p. plus attachments.
Hayes, J.B., 1964, Geodes and concretions from the Mississippian
Warsaw Formation, Keokuk region, Iowa, Illinois,
Missouri: Journal of Sedimentary Petrology, v. 34, no. 1,
p. 123-133.
Wathen, B., 1992, The gems of Hancock County: The Nature
of Illinois, Vol. VII, No. 1, p. 22-24.
Contributed by David L. Reinertsen,
D. Scott Beaty, and Jonathan H. Goodwin

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