Geodes are hollow or partially filled, roughly spherical structures, globular or oblate spheroidal discrete bodies or structures lined with mineral masses or crystals. However, some are irregular or mishappen, and others are oblong, like fossils of invertebrates.
They occur in weathered volcanic rocks, especially lava flows, and sedimentary rocks like limestone beds, dolomite, shales, and other rocks.
Geodes are less than an inch to several feet, with most being a few inches in diameter. Also, their shape and the lining or filling minerals or materials differ.
Their outer surface is rugged, rough, or pitted, often grayish to brownish. It lacks unique and interesting appeal and resembles a roughly spherical body or common rock.
However, when cut in half, they will display colorful and stunning mineral formations. Some have well-formed mineral crystals. These minerals grow inward and differ from the surrounding rock.
The name geode originates from an Ancient Greek word, geṓdēs, which means earthlike. It refers to the earth-like shape, i.e., nearly spherical.
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What do they look like?
Geodes come in various sizes, shapes, crystal formations, and colors, each unique.
1. Shape
Geodes are hard, mostly oval, nearly spherical, or globular discrete rocky structures. They are more like the shape of a ball, egg, or the Earth.
However, some are oblong, triangular, square, cylindrical, teardrop, or other more irregular shapes.
2. Size
Geodes are less than 0.1 inches to 36 inches, with some larger. Something like less a pea to walnut to football basketball or larger.
However, most are 2-6 inches in size. Some, like those in the Paraná Basin of South America, are large enough for people to hide inside.
3. Outer appearance
Their outer surface is rough, bumpy, or lumpy, and they appear gray or brownish. They look like common rocks or a mudball thin clayey film.
This outermost layer or rind is made of chalcedony, a microcrystalline quartz. Its thickness varies from a film to over an inch and isn’t related to size, and it differs in composition from the surrounding host rock.
The chalcedony that makes the rind is relatively weathering resistant than the surrounding host rocks. Therefore, geodes will remain intact as discrete bodies when the host or bedrock weathers away.
4. Inner appearance
When cut in half, the inner layers display colorful minerals and a hollow center. These include spectacular banded agate, blue-gray chalcedony, and smoky, gray to transparent quartz, some forming successive layers or concentric bands.
Also, they may have dolomite, calcite, amethysts, metal sulfides, and other well-formed or euhedral, beautiful exotic mineral formations and crystals that grow inward to the core. These minerals are well-formed since they grow unconfined into the void.
However, some don’t have well-formed, shiny, stunning, or beautiful crystals. Such will have layers of minerals like agate, quartz, opal, etc.
That is not all. Some minerals in geodes are druzy or drusy, i.e., they have small, often well-formed mineral crystals projecting inward, forming a druzy fabric.
Lastly, some geodes can have dendrites or a dendritic structure. Also, chalcedony may show botryoidal habit.
Some minerals with clear surfaces and many facets can reflect light, creating a sparkling, beautiful appearance.
Crystals and minerals in geodes
Geodes have different mineral compositions compared to the host rock. Which minerals a given geode has depends on location, host rocks, and conditions like temperature or acidity (pH).
For instance, quartz is common in igneous geodes, while silica is in those that form in sediments or sedimentary rocks.
On the other hand, those in America’s Midwest sedimentary rocks often contain minerals like dolomite, pyrite, and calcite.
Unfortunately, you cannot know the minerals present before cutting or breaking them. However, those in certain locations can have similar properties.
The most common mineral in geodes is quartz, and its varieties include colorful banded agate, jasper, chalcedony, and opal. Also, calcite, dolomite, and rich purple amethysts are often present.
Less common minerals include datolite, evenkite, celestite, millerite, marcasite/pyrite (fool’s gold), ankerite, okenite, geothite, pecoraite, malachite, whewellite, colemanite, apophyllite, strontianite, smythite, and barite.
Others are sphalerite, aragonite, hematite including gold-laced hematite, magnetite, goethite/limonite, gypsum, chalcopyrite, volkovskite, shattuckite, rhodochrosite, kaolin, smithsonite, gem silica (like blue or stalactitic)
Besides minerals, some may have water or liquid petroleum (bitumen). Some geodes, such as those in Illinois, have enough petroleum that squirts as you break them.
In most cases, minerals like calcite, chalcedony, and agate tend to occur towards the outer regions, and metal sulfides and quartz crystals towards the center.
Lastly, the most valued and sought-after geodes are those with amethyst crystals or black calcite.
How are geodes formed?
Geodes form when mineral-rich rain, groundwater, or hydrothermal fluids percolate and crystallize or precipitate minerals inside cavities. This process takes thousands to millions of years.
Cavities in geodes of volcanic origin form when solidifying lava traps that exsolve volatiles.
Those in sedimentary rocks from the dissolution of nodules or concretions. Also, voids could be from animal burrows in sediments or decay roots, branches, fossil remains like shells, etc., entrapped in sediments or sedimentary rocks.
We have an exclusive article on how geodes are formed in nature that has the nitty-gritty details.
Geode colors
Some geodes’ breathtaking coloration includes tan, blue, purple, red, cream, colorless, rich purple hues, etc. However, the browns, blues, and whites are the most common.
These colors often occur in the banded agate and concentric mineral layers. Also, mineral crystals inside geodes will show colorful formations.
We all want the colorful ones. Unfortunately, there is no way of knowing the colors they have before opening them.
The color of geodes depends on chemistry and impurities soaked in mineral-rich fluids precipitating or forming these structures. Some impurities that impart color include iron, cobalt, magnesium, nickel, chromium, titanium, manganese, etc.
For instance, titanium will create a blue coloration. Manganese will produce a pinkish hue, and chrome or nickel will make a green color.
On the other hand, iron will give a red, purple, or rusty appearance, while cobalt will result in a red color. The deep purple in amethyst is from iron impurities.
Porosity and the dying solution will influence how well mineral layers or crystals absorb color. For instance, chalcedony and agate are more porous than pure quartz. Thus, they tend to take more colors.
Also, changes in conditions like temperature can alter color. For instance, heating the rich purple amethysts will produce soft yellow citrine.
Lastly, some unusual, intense, bright, or vibrant geodes are often dyed with various color pigments.
Where are geodes found?
Geodes occur in places with weathered volcanic lava, ash, and tuff or sedimentary rocks like limestone, dolomite, and sometimes shale or other rocks. Other diverse environments can have a small volume.
In volcanic zones, they are common in places with stratified basalts and basaltic tuff. But can occur in other volcanic rocks, including rhyolite flows.
On the other hand, those in sedimentary rocks occur in stratified sedimentary carbonates like dolomite and limestone beds. Some can occur in shale and other rocks.
Usually, geodes are more common in desert regions. However, they can occur in other places too. You will find them in stream valleys, soil, or sediments formed from weathered rocks, canyons, or scattered.
1. Where they occur in the us
In the US, geodes are common in the Midwest and lower Midwest states. These states include Iowa, Indiana, California, Arizona, Utah, Nevada, Illinois, Missouri, and Kentucky. Idaho, Colorado, Montana, Kansas, Wyoming, Wisconsin, New York, and South Dakota.
Some of the famous places with geodes in the US include
- Keokuk in Iowa in the limestones and dolomites of Warsaw Formation
- Dugway Geode beds in Juab County, Utah. Dugway also covers parts of Illinois and Missouri.
- Warsaw, Hamilton, and Nauvoo in Illinois
- Warsaw-Salem and Fort Payne formation and Green River in Kentucky
- Harrodsburg Limestone and Ramp Creek Formation in Indiana
- Woodbury in Tennessee, among the Warsaw Formation limestone and dolostones
- Potato Patch, Cinnamon, Hauser, and North Black Hill Geode Beds in California
- Flint Hills in Cowley County, north of Douglass in Butler County, and in Marshall, Riley, and Chase counties in Kansas
- Lake Superior in Wisconsin
- Sugarloaf Mountains in Maine
- Serpent Mound Zinc Deposit and Findlay Arch Mining District in Ohio
- Whitewater, Wolf Creek Pass, and Felch Creek in Colorado
2. Elsewhere
Outside the US, they occur in Brazil, Namibia, Uruguay, Australia, Spain, and Mexico. In Mexico, they are called coconut geodes and are awesome specimens.
Also, they occur in Mendip Hills in England, commonly called potato stones, in Saudi Arabia, Iraq, Orman, Madagascar, Australia, China, India, and Kuwait.
What are geodes used for?
Geodes have some of the most attractive mineral formations, including colorful agate linings and gemmy crystals. This makes them a highly valued treasure by collectors, rockhounds, science museums, education and science departments, etc.
Uses will depend on appearance and size. Some of the uses of geodes include:
1. Making jewelry
Smaller, crystal-rich geodes are cut, polished, and used to make parts of jewelry like pendants in necklaces, brooches, etc.
Also, some are a source of minerals for making jewelry, while the gold-laced hematite found in geodes from central Spain makes magnetic jewelry or decorative magnets.
2. Display specimens
The colorful gemmy or banded geodes, like those with agate or amethysts, are cut as display specimens for students and enthusiasts.
3. They make decorative ornaments or items
Beautiful geodes are cut and polished as decorative ornaments for homes and offices. They also make bookends, clock faces, desk sets, clock faces, wind chimes, coasters, etc.
Interior designers price large ones as they make various objects of art, especially colorful ones like those with amethysts. Also, they are carved to make bathtubs.
4. Source of revenue or profits
Geodes are a source of livelihood or profitable local industry to communities where they occur, like Brazil, Namibia, Uruguay, and Mexico.
These communities collect and sell these geodes to gem and mineral shows, science museums, rock shops, art galleries, novelty shops, science and education departments, tourists, etc.
Some are sold for opening experience. Such an event often attracts many enthusiasts.
Frequently Asked Questions (FAQs)
The largest geode in the world is the Geode of Pulpi. It is located near Pulpi Town in Almeria, Spain. Its volume is 10.7 cubic meters. Its average size is 8 m long by 1.8m wide by 1.7 meters high.
Crystal caves (informal) refer to extremely large geodes lined with large crystals. It may also refer to specific geological heritage locations like Crystal Cave in Ohio or Cave of Crystals in Mexico,
Geodes are opened by sawing a diamond or carbide rock saw or cutting them with a pipe cutter or geode slicer. Also, you can tap and shatter them with a rock hammer or a hammer and chisel. Remember to use an appropriate surface, as they can damage tables or floors. Also, when breaking them, wear protective clothing and take care as fragments can fly, causing injury, and cover them with a towel,
These are pyroclastic materials deposited from explosive volcanic eruptions that bear geodes. Examples occur in volcanic tuff in the Dugway Mountains in Utah, USA.
Dull and less attractive geodes like grays and whites are often dyed with bright and vibrant colors like green, purple, red, blue, etc., to improve their appearance. Dying involves soaking them in colored organic or inorganic solutions. An example is onyx, a dyed agate geode.
Go for the lightweight, i.e., not fully filled, as they tend to have well-formed minerals. However, there is no way to know what minerals they have or their colorations unless opened.
Geodes are named according to their most dominant minerals, the features they exhibit, or their place of origin. You will find names like quartz, agate, Oco, coconut, Keokuk, Dugway, Woodbury, Brazilian, or Amethysts geodes.
Yes. There are man-made or fake geodes. Some are made with decorative clay and then lined by glued galena. They look like natural ones and can easily be confused by some gemologists, mineral collectors, or paleontologists.