Thundereggs vs geodes

Thundereggs vs geodes DEFAULT

If you’ve ever been to a gem and mineral show or a store that sells rocks, you’ve definitely seen geodes or thundereggs. These beautiful polished and cut rocks are usually displayed prominently to catch a viewers eye.

They can be displayed alone, or used as book ends or paperweights. They come in all sorts of colors, shapes, and sizes. Usually they have beautiful crystals formed on the inside in intricate patterns or shapes.

Thundereggs vs Geodes

thunderegg vs geode

Thundereggs and Geodes are two types of rocks prized by rockhounds and mineral collectors for their plain rocky outsides with beautiful crystal or crystalline insides. If both rocks are plain on the outside with beautiful treasures inside, then what are the differences between the two?

Is the difference decided by geologists or rockhounds? It turns out, there IS a specific difference between thundereggs and geodes! 

A geologist defines a thunderegg as having formed in a volcanic rhyolite rock layer and is a structural formation. Conversely, a geode can occur in both volcanic and sedimentary rocks. This makes every thunderegg a type of geode by definition, but not every geode is a thunderegg! And even that is up for debate!

Controversy in the Geologic Community

There is controversy and debate on thunderegg classification as a geode, and different geologists will give you different answers. 

Void Space and Shapes

Some geologists say that a geode must have a void space, otherwise it should be designated as a “nodule,” while others say that they must be spherical or sub spherical. 

Not all Voids are Created Equal

Within the geode/thunderegg controversy, method of formation is another big argument. Thunderegg void spaces are most prized when they contain points, which is believed to be due to a fracture pattern. Due to the method of void space creation, some geologists do not classify fractures as vugs and so they deem the thunderegg different from a geode. 

Depends Who You Ask

Ultimately, the answer depends on who you ask. Both are secondary structures with secondary mineralization. I have found in my years of touring and shopping at gem and mineral shows that the thundereggs are always included with the geodes because of their similarities.  With the knowledge of what is a geode and what is a thunderegg you can decide if you would put the two together on your shelf.

What is a Geode?

what is a geode vs thunderegg

A geode is a rock structure that forms around a void, or open space, that becomes filled with crystals or crystalline deposits over time. These voids are known as”vugs” to geologists. If we remember back to the three types of rocks we know there are igneous rocks, metamorphic rocks, and sedimentary rocks.

Geodes form in voids, like gas bubbles, in igneous rocks, and in void spaces left by erosion in sedimentary rocks. Metamorphic rocks do not often contain void spaces due to the folding, compression, and melting of the rocks.

Secondary Crystal Formation 

Now that we have a method for creating a hollow void space, water does the work of transporting mineral rich fluid into these spaces. Over time, the minerals in the ground water are deposited in the space and grow into beautiful crystals of all types.

Crystals can be quartz, calcite, barite, gypsum, or any other multitude or combination of crystals if the required elements are found in the local ground water! 

breaking open geode with hammer

How Geodes Make it to the Surface

Once the harder mineralized inside forms, the softer host rock around it can erode leaving the spherical nodule of crystals that can be cut in half to reveal the crystal growth inside. Sometimes you can even tell which direction was up when the minerals precipitate out by identifying a flat layer that was formerly the sitting waters surface!

What Is a Thunderegg?

what is a thunderegg vs geode

A thunderegg is a geologic structure that forms specifically in rhyolite, an igneous type of rock that comes from volcanic activity. Rhyolite is a silica rich volcanic rock that is very commonly associated with big, violent, volcanic explosions! As it gets deposited from volcanic eruptions it creates layers that trap gasses, and when it cools it contracts to create fractures and voids, perfect spaces for forming crystals.

Thunderegg Crystal Formation

Now that a void is formed, how do crystals get deposited inside? Mineral rich ground water again comes into these structures over time and slowly precipitates out layers of fine cryptocrystalline quartz, also known as chalcedony. But chalcedony isn’t the only thing we find in thundereggs.

Quartz crystals, gypsum crystals, and even zeolites can be found in these voids. Just like a geode, the process for crystal formation is precipitation, but the specific host rock type and location makes the structure a thunderegg. 

thundereggs

Why Do They Have Different Names?

Now you might be wondering, “Why is there a difference between calling one a geode and one a thunderegg?” Besides the structural differences, the term thunderegg actually comes from history.

Native Americans in the Pacific Northwest were the first to find and name these beautiful stones “Thundereggs.” They held the belief that the thundereggs were the eggs of thunderbirds, or thunder spirits, and that they were formed by the storms and the thunder and lightning caused between battling thunder spirits who hurled their eggs at each other from mountain top to mountain top!

Where can I find Geodes and Thundereggs?

large thunderegg from richardson rock ranch

Geodes and thundereggs can be found all over the world, but there are many places that are famous for collectors due to the abundance and/or the quality and types of geodes that can be found.

There are many famous localities where these beautiful stones are mined or collected. The following are some of the most famous geode and thunderegg locations around the world.

Brazil

Brazil is famous for their geode deposits. Large basalt flows in Brazil contain huge vugs that range from an inch or two to being so large you can stand inside them. The basalt vugs are full of beautiful purple amethyst crystals and some even contain secondary crystals like calcite that grow on the amethyst.

These geodes can range from just a few dollars to tens or even hundreds of thousands of dollars. Though the area is famous for the large geodes, the extraction process is very intensive and requires heavy machinery for the hard basalt host rock. These mines are not open to casual collectors, but samples are commonly for sale all over.

Keokuk Geodes

American geodes are very common online and at rock shows available to crack open yourself. The Keokuk region geodes are small enough to fit in your hand, or sometimes larger than your head. They originate in mudstones near the intersection of Missouri, Iowa, and Illinois and are found in-situ (in the rock formation) or weathered out and found on the ground or in stream beds.

The geodes are formed in the Warsaw Formation, a roughly ~340 million year old mudstone formation. The Keokuk geodes are predominantly quartz, but are also known to contain sulphites such as pyrite or sphalerite. These geodes can be collected by rockhounds or you can even buy them online and try your hand at cracking them open at home.

Dugway Geodes

has their own famous geodes that can be found in surface level sediments just a short drive southwest of Salt Lake City. Juab County, , has the Dugway Geode beds which are also rhyolitic in composition.

These geodes look completely different from the Pacific Northwest thundereggs and are not considered the same thing. These geodes commonly have strange shapes and are often light blue and pale pink with agate, chalcedony, and druzy quartz interiors.

Some are even found to fluoresce in ultraviolet light due to trace amounts of uranium. These geodes can be found by collectors in person by anyone who wants to search for them. Maps to the Dugway Geode beds can be obtained from www.Geology.utah.gov.

Thundereggs

American thundereggs are famous for being found in the Pacific Northwest near the Cascade Mountain Range. Oregon in particular is famous for having thundereggs and even designated it their official state rock.

Crook, Jefferson, and Wheeler counties are known to have the largest deposits of thundereggs and they can be found in roadside highway cuts, river cuts in the area, or weathered out and found in the soils near the rhyolite flows.

There are several rock shops in the area that can direct you to local hunting spots where thundereggs can be found in the rhyolite deposits. The Ore Bin, vol. 27 from 1965 will provide a more detailed history along with maps and geologic information about specific formations that contain thundereggs.

Counterfeit Geodes

Rockhounds beware! Due to the beauty and desirability of geodes and thundereggs there is a whole industry devoted to created fake geodes and thundereggs. Many geodes are dyed strange colors and the seller will not be honest about the origins.

Other fakes include gluing accessory minerals that do not form in geodes naturally. Some industrious folks have even learned how to grow non natural crystals into fake rocks they make out of plaster.

Please always be aware that not all geodes for sale are 100% natural, and even though the man made ones may be beautiful as well, they may be sold under false pretenses as 100% natural. 

Sours: https://www.rockseeker.com/thunderegg-vs-geode/

Occurrence

Thunder Egg

A Thunder Egg “Thunderegg” is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers. Thundereggs are rough spheres, most about the size of a baseball—though they can range from less than an inch to over a meter across. They usually contain centres of chalcedony which may have been fractured followed by deposition of agate, jasper or opal, either uniquely or in combination. Also frequently encountered are quartz and gypsum crystals, as well as various other mineral growths and inclusions. Thundereggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colours. A characteristic feature of thundereggs is that (like other agates) the individual beds they come from can vary in appearance, though they can maintain a certain specific identity within them.

Thunder Egg is not synonymous with either geode or agate. A geode is a simple term for a rock with a hollow in it, often with crystal formation/growth. A thunderegg on the other hand is a specific geological structure. A thunderegg may be referred to as a geode if it has a hollow in it (see illustration of Gehlberg specimen), but not all geodes are thundereggs because there are many different ways for a hollow to form. Similarly, a thunderegg is just one of the forms that agate can assume.

Many thundereggs found at Rockhound State Park are spherical and consist of two distinct parts: a dark-gray to pinkish outer part and a white, blue, or gray inner part, or core, which is recognizable as agate, chalcedony, and quartz crystals, all forms of the compound SiO2. In many examples, these two parts can be described as a shell and a filling. However, some thundereggs, or spherulites, do not contain the filling; they are composed of solid dark-gray to pinkish shell material (Fig. 3) or are partly hollow. Geologically distinct processes form the two parts of the thundereggs. The outer part of the thundereggs is formed by complex magmatic processes (i.e. as spherulites), and then the inner part is formed and modified by multiple cycles of late-stage hydrothermal fluids. The processes that form geodes and thundereggs are complex and are controlled by constantly changing physical and chemical conditions, such as temperature, pressure, depth of formation, composition of the magma, composition of the ground water, and composition of the host rocks.

Thundereggs are found globally wherever conditions are right. In the USA, Oregon remains one of the most famous thunderegg locations. Germany is also an important center for thunderegg agates (especially sites like St Egidien and Gehlberg). Other countries known for their thundereggs include some places in Africa, Poland, Romania, Turkey, Mexico, Argentina, Canada, Australia and France.

Thundereggs are found in flows of rhyolite lava. They form in gas pockets in the lava, which act as molds, from the action of water percolating through the porous rock carrying silica in solution. The cooled bubbles were gradually filled by water percolating through the porous rock carrying rich quantities of silica (quartz). The deposits lined and filled the cavity, first with a darker matrix material, then an inner core of agate or chalcedony. The various colors come from differences in the minerals found in the soil and rock that the water has moved through.

The agate, chalcedony, and quartz veins and open-space-fillings within voids in the spherulites formed later by multiple cycles of hydrothermal fluids. Hydrothermal fluids are a mixture of late-stage fluids escaping the magma and local ground water. The fluids contain some elements from the original magma and also dissolved minerals from the country rock. The amount of ions that the fluid can dissolve depends upon pH, temperature, pressure, and composition of the fluid. The hydrothermal fluids move through fractures in the rocks, which crosscut the igneous textures, and form veins or banded agate, chalcedony, and quartz. Some of these fluids seep through microscopic pores and into spherulites and gas pockets in the volcanic rocks, and they precipitate crystals along the walls of the cavity, forming geodes and geode-like spherulites. Other fluids seep through fractures and gas and other void spaces in the spherulites. Because of their formation by multiple hydrothermal events, each thunderegg provides clues as to its unique formation.

Different temperatures and fluid compositions would account for the variety of textures found within any given thunderegg or geode. By carefully studying the crystal fill and textures in spherulites and geodes, geologists can piece together the different processes through time that formed them. The banding found within some spherulites and geodes consists of multiple layers of different colored agate, chalcedony, and locally quartz, and may have been formed by fluids supersaturated in silica (Fournier, 1985a). Supersaturated solutions are solutions that contain excessive silica in solution. The presence of silica minerals within the thundereggs and geodes indicates that the fluids were saturated in silica. Saturated fluids are fluids that contain enough silica in solution without precipitating silica minerals. When a silica-saturated solution cools slowly, crystalline quartz is deposited at approximately 200°–340° C (Fournier, 1985a). Rapid cooling of a silica-saturated fluid allows supersaturated solutions to form that precipitate chalcedony or amorphous silica. These supersaturated fluids are unstable and quickly deposit thin layers of chalcedony or amorphous silica, typically at lower temperatures (<200° C). The fluid loses silica due to precipitation and becomes saturated with silica but as the fluid continues to cool rapidly, it becomes supersaturated with silica again. An increase in salinity (such as NaCl) increases the solubility of silica at higher temperatures and also produces saturated silica solutions (Fournier, 1985a). Super­saturation of the fluids can also occur by mixing of different hydrothermal fluids, especially with different pHs, and by reaction of hydrothermal fluids with volcanic gases.

The different colors of the bands are a result of trace amounts of impurities, such as iron (red), manganese (black, pink), cobalt (blue, violet-red), copper (green, blue), chromium (orange-red), nickel (green), etc. Faceted quartz crystals indicate that the fluids were somewhat supersaturated with silica and that precipitation occurred under relatively slow-changing conditions (Fournier, 1985a).

Not all geodes are spherulites formed by magmatic processes; other natural processes form some geodes. Lower-temperature ground water percolates through the cooled volcanic rocks and dissolves additional minerals. These fluids are typically low temperature (<200° C), although locally higher-temperature (200°– 300° C) ground waters may be present, especially adjacent to the volcanic vents. These fluids move through microscopic pores in the rock by a process called “diffusion.” During diffusion, some ions in the fluid collect into void spaces and gas pockets in the rock, whereas other ions cannot pass through. This collection of ions surrounding these void spaces may actually, in some cases, form the hard outer shell that is characteristic of geodes. The outer shell may be strengthened by the precipitation of some ions that were excluded during crystallization of agate, chalcedony, and quartz and concentrated in the remaining fluid. Fluids also enter the void spaces through fractures. The void spaces may have originally formed by gas pockets within the magma or by prior dissolution of spherulites or other features within the volcanic rock.

How do the Thunder Egg/spherulites become hollow?
this is difficult to demonstrate directly, we speculate that the hollow centers of spherulites are formed by nucleation, coalescence, and expansion of vapor bubbles at high temperature, resulting in a hollow center that can be filled later by silica. The vapor bubbles would have formed as a result of crystallization of quartz, feldspar, and magnetite, which contain no water, from rhyolitic magma, which contains a small amount of dissolved water at atmospheric pressure (Taylor, 1986). Calculations of the volume of water vapor that could have formed from anhydrous crystallization suggest that the volume would be more than enough to generate the size of hollow cavities seen in spherulites. The reason that some spherulites are hollow and others are not may be related to the rate and depth of crystallization and to the resultant ability or inability of vapor bubbles to nucleate and coalesce.

Reference:
Wikipedia: Thunderegg
Oregon State Rock:State Symbols USA
The Formation of Thundereggs: Robert Paul Colburn PDF
Rock Hound State Park and Spring Canyon Recreation Area, Recreation Area: New Mexico Geology, v. 22, no. 3, p. 66-71, 86.

Geology Page

Sours: https://www.geologypage.com/2016/10/thunderegg.html
  1. Fortnite brutus shadow
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  3. Glass handrail bracket
  4. German powerpoints
  5. Montego curtain

What’s the Difference Between Thunder Eggs and Geodes?

This thunder egg from Haida Gwaii, called Star Spirit, has a solid interior of blue and grey chalcedony.  

Thunder eggs and geodes are sought-after by collectors and novices alike. Both are round and reveal beautiful interiors, however, there are some distinct differences between these two types of specimens.

Thunder eggs, which are technically known as lithophysae are typically found with solid or near-solid cores of chalcedony or agate, whereas geodes are relatively hollow inside with thinner layers of quartz crystals. 

This smaller geode has a quartz crystal centre with a small amount of amethyst colouring. Blue banded agate surrounds the crystal interior. This specimen is from Haida Gwaii.&nbsp;
This geode from Haida Gwaii has an empty core and its inner layer is covered with small amethyst crystals.&nbsp;
These crystal clear Fairy Geodes were cut at Crystal Cabin in Haida Gwaii.&nbsp;

Thunder Egg Formation

Thunder eggs are spherical concretions of solid or near-solid nodules filled with chalcedony, agate or quartz crystals. Crystal Cabin's Star Spirit Thunder Eggs from Haida Gwaii are formed in ash beds in the top sections of silica-rich rhyolitic lava flows. Generally, the formation of thunder eggs seems to be more common in the gas-charged ignimbrite eruptions.

Star Spirit Thunder Egg from Haida Gwaii
The exterior of a Star Spirit Thunder Egg.&nbsp;

As volcanic lava cools, a shell forms within the rhyolite ignimbrite around trapped gas bubbles. Flow-lines found throughout the ignimbrite are preserved in the shell. Compaction over millions of years causes these gas bubbles to eventually crack the rhyolitic shell and allows the gas or liquid to escape. This leaves behind a hollow cavity. Heated ground water leaches silica and other impurities from volcanic ash and deposits them into the cavity. The watery solution slowly turns to gel, but first creates a silica layer in the innermost core. The gel eventually hardens to deposit chalcedony, agate, quartz crystals, or a combination of these minerals. These minerals form the core of a thunder egg.

Sours: https://www.crystalcabingallery.com/blogs/news/what-s-the-difference-between-thunder-eggs-and-geodes
Thunder Eggs

Thunderegg: Ordinary Rocks With Crystal Surprise Inside!

Thunderegg. Credit: Bill The Eggman



A thunderegg (or thunder egg) is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers.

Thundereggs are rough spheres, most about the size of a baseball—though they can range from a little more than a centimeter to over a meter across. They usually contain centres of chalcedony which may have been fractured followed by deposition of agate, jasper or opal, either uniquely or in combination.

Thundereggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colours. A characteristic feature of thundereggs is that (like other agates) the individual beds they come from can vary in appearance, though they can maintain a certain specific identity within them.

Thunderegg Vs. Geode

Thunderegg is not synonymous with either geode or agate. A geode is a simple term for a rock with a hollow in it, often with crystal formation/growth. A thunderegg on the other hand is a specific geological structure. A thunderegg may be referred to as a geode if it has a hollow in it, but not all geodes are thundereggs because there are many different ways for a hollow to form. Similarly, a thunderegg is just one of the forms that agate can assume.

Formation

Thundereggs are found in flows of rhyolite lava. They form in gas pockets in the lava, which act as molds, from the action of water percolating through the porous rock carrying silica in solution. The cooled bubbles were gradually filled by water percolating through the porous rock carrying rich quantities of silica (quartz).



The deposits lined and filled the cavity, first with a darker matrix material, then an inner core of agate or chalcedony. The various colors come from differences in the minerals found in the soil and rock that the water has moved through.

The agate, chalcedony, and quartz veins and open-space-fillings within voids in the spherulites formed later by multiple cycles of hydrothermal fluids. Hydrothermal fluids are a mixture of late-stage fluids escaping the magma and local ground water.

The fluids contain some elements from the original magma and also dissolved minerals from the country rock. The amount of ions that the fluid can dissolve depends upon pH, temperature, pressure, and composition of the fluid. The hydrothermal fluids move through fractures in the rocks, which crosscut the igneous textures, and form veins or banded agate, chalcedony, and quartz.

Some of these fluids seep through microscopic pores and into spherulites and gas pockets in the volcanic rocks, and they precipitate crystals along the walls of the cavity, forming geodes and geode-like spherulites. Other fluids seep through fractures and gas and other void spaces in the spherulites. Because of their formation by multiple hydrothermal events, each thunderegg provides clues as to its unique formation.
Credit: John Meinen

How to Spot Thundereggs?

Though thundereggs appear as rocks, they can be easily identifiable by the abnormally round and bumpy brown-gray exteriors. Usually, thundereggs lie relatively close to the Earth's surface, embedded in the clay of the tuff, where they were formed. During the time, the tuff has been decayed to soft mud and clay, making rock-like thundereggs noticeable.

Where to Find Thundereggs?

Thundereggs are found globally where conditions are optimal. In the US, Oregon is one of the most famous thunderegg locations. The regions of Central and Eastern Oregon are the most popular in the world for thundereggs hunting. The best productive areas are located in the high desert of Central Oregon near Prineville, Madras and in the Ochoco National Forest (Central Oregon). Also, thundereggs can be collected in Succor Creek Creek Canyon (Eastern Oregon)
  • White Fir Springs
  • White Rock Springs
  • Whistler Springs
  • Richardson Rock Ranch
  • Succor Creek Canyon
  • Alvord Desert
  • Owyhee Reservoir 

See also:

Where to Find Sunstone in Oregon?

Dig Your Own Unique Opals From Nevada

Opals in Oregon

Sours: https://www.geologyin.com/2020/05/thunderegg-ordinary-rocks-with-crystal.html

Geodes thundereggs vs

GeneralThunder eggs vs. geodes

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There are/were several discussions relating to geodes recently. Mindat has a page on thunder eggs, and the discussion on that page indicates they are similar to geodes. Other discussions here suggest it may be OK to consider thunder eggs to be a type of geode, but that would not seem to be a unanimous opinion. Mindat lists only a few localities for thunder eggs world-wide. As a former resident of both California and Oregon, I know that many people in that part of the country collect them, and I seem to remember there were many places in western states where they could be collected. I saw hundreds and hundreds of thunder eggs in exhibits, museums, mineral shows, and private collections when I lived out there, but hardly any Indiana/Keokuk type geodes. Therefore I find it odd that there is so little discussion of them on Mindat compared with geodes, and so few localities lissted. Because Midwestern-type geodes occur in sedimentary rocks, and thunder eggs occur in volcanic rocks, it seems there would be some value in labeling them differently. In other words, the different labels would indeed paint different pictures, refer to different geological circumstances, and even suggest different parts of the country/world. Certainly this has already been sorted out by someone, somewhere–Yes, No?

5th Jan 2013 13:37 UTCBob Harman

For me all these threads discussing what is a geode and what is not boils down to both formal geologic definitions such as Harold M stated in the other thread and what collectors call the specimen that thay are looking at. It is like pornography no one can precisely define it, but you know it when you see it. It means different things to different people. A rose is a rose is a rose so to speak.


Midwest geode collectors, to my knowledge, will not consider agatized corals as geodes. Western thunder eggs will be igneous "first cousins" of geodes. Madagascar celestine "geodes" will be geodes even tho they have celestine rather than quartz rinds. And the great South Dakota concretions with calcite and barite will not be considered a geode even tho they weather out of the softer surrounding matrix and are hollow with calcite crystals lining the cavity and secondary barite minerals often present. Collectors most often consider them as concretions


So formal dictionary definitions and related not withstanding, a geode is what you consider a geode. There is the very broad definition and narrower definitions. A geode is a geode is a geode. Other concretions and nodules and cavities and gas bubble vesicles and agatized corals are close "cousins" but not geodes. So that is my informal take on all this. CHEERS..................BOB

5th Jan 2013 17:19 UTCDavid Zimmerman (2)

Hi Norman,


I still think the terms need to conjure a specific image when said. Why would thundereggs "want" to be considered ordinary geodes....because they certainly are not. They have their own special moniker and should conjure up their own special image in ones head. . The same can be said about septarian nodules and Tampa Bay fossilized coral. I differ from Bob because I do consider igneous amygdeloids to be true geodes, as do I consider the celestine geodes of Madagascar and also the calcerous geodes from Keokuk region.


Using the strict definition that others posted in the other thread about what a true geode is, then many types of druzy agates would be considered geodes, but once again, I think they have their own special moniker that supersedes a "geode" moniker.


No one ever accused me of being a purist!

5th Jan 2013 22:25 UTCRalph Bottrill 🌟Manager

Personally, I have a bit of a problem with things like thunder eggs and geodes being given mineral pages of their own, as they are just curious rock formations, despite being often near monominerallic; these should really be in the glossary until we get a Rockdat? Or under agate, quartz, etc. Else we should have mineral pages for stalactites, vesicles, concretions, pseudomorphs, etc. Geode is a much-abused term but should just imply a rounded, hollow, or once-hollow, mineralized geological structure. Thunder eggs are a bit different to sedimentary geodes or vesicle-filling geodes but still fit the definition if you inclde the host volcanic spherulite part of the structure, and could be considered an agate-filled subtype, and of course there are numerous names for all the agate varieties.it just seems more sensible to classify them by the mineral name than the geological structure.

6th Jan 2013 04:21 UTCNorman King 🌟Expert

Thanks for the responses. I'm surprised about the rudimentary state of nomenclature for these objects. In my own field of sedimentology I usually can be heard complaining about too much terminology and of inconsistent or contradictory systems of nomenclature. But those systems are generally quite detailed and specific. There seems to be a bit of a vacuum about geodes. Maybe discussion of geode-related terminology will be more likely on Gemdat. Someone (or group) should publish a paper on that terminology to lay out a system that can be referred to as a standard. Unfortunately, there is probably little incentive in academia for that, and unfortunately also the subject seems to cross discipline boundaries, involving sedimentary and igneous rocks, and the secondary (diagenetic) processes are likely to be rather different.

26th Jan 2013 17:38 UTCJohn Davis (2)

I am a collector from Indiana and am quite familiar with the midwest geodes. But I also collect thunder eggs from the south west and have many of both in my collection. It has always been my understanding that the definition of a geode is to have a outer shell, hollow and have an inward pointing crystal structure. If this is the true definition of a geode. A thunder egg that has a open cavity and has crystals growing on the inside, would definately fall into the geode classification. In my collection, I refer these to thunder egg geodes to seperate them from the sedimentary geodes of the midwest.

 

Sours: https://www.mindat.org/mesg-281105.html
What is the difference between a Geode and a Thunder Egg?

Thunderegg

This article is about a type of rock. For the rock and roll band, see Thunderegg (band).

Nodule-like rock, that is formed within rhyolitic volcanic ash layers

A thunderegg (or thunder egg) is a nodule-like rock, similar to a filled geode, that is formed within rhyoliticvolcanic ash layers.[1] Thundereggs are rough spheres, most about the size of a baseball—though they can range from a little more than a centimeter to over a meter across. They usually contain centres of chalcedony which may have been fractured followed by deposition of agate, jasper or opal,[1] either uniquely or in combination. Also frequently encountered are quartz and gypsum crystals, as well as various other mineral growths and inclusions. Thundereggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colours. A characteristic feature of thundereggs is that (like other agates) the individual beds they come from can vary in appearance, though they can maintain a certain specific identity within them.

Thunderegg is not synonymous with either geode or agate. A geode is a simple term for a rock with a hollow in it, often with crystal formation/growth. A thunderegg on the other hand is a specific geological structure. A thunderegg may be referred to as a geode if it has a hollow in it, but not all geodes are thundereggs because there are many different ways for a hollow to form. Similarly, a thunderegg is just one of the forms that agate can assume.

Occurrence[edit]

Thundereggs are found globally where conditions are optimal. In the US, Oregon is one of the most famous thunderegg locations. Germany is also an important center for thunderegg agates (especially sites like St Egidien and Gehlberg). Other places known for thundereggs include Ethiopia,[2]Poland,[3]Romania, Turkey, Mexico, Argentina,[4]Canada, Mount Hay[5] and Tamborine Mountain,[6] (Australia) and the Esterel massif,[7] (France).[citation needed]

Formation[edit]

Thundereggs are found in flows of rhyolite lava. They form in the lava from the action of water percolating through the porous rock carrying silica in solution. The deposits lined and filled the cavity, first with a darker matrix material, then an inner core of agate or chalcedony. The various colors come from differences in the minerals found in the soil and rock that the water has moved through.[8]

State rock designation[edit]

On March 30, 1965, the thunderegg was designated as the Oregon state rock by a joint resolution of the Oregon Legislative Assembly.[9][10][11] While thundereggs can be collected all over Oregon, the largest deposits are found in Crook, Jefferson, Malheur, Wasco and Wheeler counties.[12] The world's largest thunderegg, a 1.75 ton specimen, is housed by the Rice Northwest Museum of Rocks and Minerals in Hillsboro, Oregon.[13]

Legend[edit]

Native American legend reportedly considers the rocks to be the eggs of the thunderbirds which occupied Mount Hood and Mount Jefferson. Thunder Spirits on the mountains hurled the "eggs" at each other.[11]

Images[edit]

  • A thunderegg agate from Falen Tree, Oregon, USA.

  • A jasper thunderegg from White Fir Springs, Oregon

  • A Friend Ranch thunderegg from Oregon

  • A thunderegg geode from Gehlberg, Germany

  • A thunderegg agate from the south of France

  • Two double thundereggs from Richardson Ranch (was known as Priday Ranch), Madras, Oregon

  • Largest opal filled thunderegg in the world, from Opal Butte in Oregon

See also[edit]

References[edit]

External links[edit]

Sours: https://en.wikipedia.org/wiki/Thunderegg

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So what's the difference between thunder eggs and geodes.  The lady at the Rock Hound state park in NM said geodes are hollow with crystals and thunder eggs are solid.  Problem was she had numerous rocks labeled as thunder eggs that had voids in them. I tried Wikipedia for help. "A thunderegg is a specific geological structure. A thunderegg may be referred to as a geode if it has a hollow in it, but not all geodes are thundereggs because there are many different ways for a hollow to form.  Okay, so not all geodes are thunder eggs, but just because a thunder egg has a hollow doesn't preclude it from still being a thunder egg.  I guess the geological structure or formation process is the key.  Okay, here are some pictures of a geode and thunder egg?  Interesting-facts-about-geodesThe thunder egg was found at NM Rockhound State Park.  Yes they let you dig in a state park for rocks.
I found this thunder egg in the park.  It has a solid quartz center (Needs some polishing)

I found these nodules near the NM AZ border.  I suspected they might be geodes.
Geode with Tabular quartz crystal
Geode with dirty center (chalcedony)
Geode? No void/vug
Geode with tinted green Chalcedony

These have a hollow inside with what appear to be chalcedony





Sours: http://rockhoundingkw.blogspot.com/2011/10/thunder-eggs-vs-geodes.html


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