Geodes
RocksForKids.com
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geodes are sphere-shaped stones, usually at least partially hollow
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geodes are often
lined inside with sparkling mineral crystals or concentric layers of
minerals
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as river beds erode, geodes tumble into streams
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Kentucky, USA
This geode half is lined with clear
quartz crystal tips, or terminations. They sparkle. The geode “skin” is
a smooth, light gray rock.
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Amethyst lined geode
Mexico
To see how this image of geode has been used in a video about fractals,
check out http://youtu.be/93akxnQ1xxw
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Keokuk, Iowa USA |
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Calcite Geode
Ohio USA |

Blue Celestite Geode
MADAGASCAR
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Dolomite "geode", Dolomite, Strontianite, Marcasite
Manitoulin Island, Ontario, CANADA
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Quartz Geode
MEXICO |
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Botryoidal Quartz Geodes
Kentucky, USA |

Quartz Geode
Kentucky, USA |
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Botryoidal Chalcedony Geode
Kentucky, USA |

The hollow spherical geode section is
partially filled in on the inside with smooth, beige, small
botryoidal chalcedony. It looks like rounded gray peas. |
| Quartz Geode from
Hall's Gap, Lincoln County, Kentucky USA |
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Locations & types
- Botryoidal Chalcedony Quartz Geode from South Fork Creek, Hall's Gap,
Lincoln County, Kentucky USA
- Quartz Crystal Geode from Hall's Gap, Lincoln County, Kentucky USA
- "Keokuk" Geode from Scheffler's Geode Mine, Alexandria, Missouri USA
Formation theory #1 - Brazil
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found in ancient volcanic rock filled with
cavities produced by steam, later filled when solutions of seeping water
found there way in & formed quartz crystals or agate
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quartz is the most common material found in
geodes
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colour depends on what impurities were
picked up by the solutions
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if the solution is deposited slowly & has
relatively few impurities, it results in crystal lined cavities
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if solution is deposited rapidly, cavity is
filled with bands of fine-grained quartz - namely agate
Formation theory #2
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they were originally nodules of limestone
or anhydrite
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formed within a soft sediment by concentric
outward growth around small nucleus or core
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the interior of the concretion washed out,
causing a hollow
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groundwater solutions then left behind a
replacement of the geode walls & the crystals inside
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takes 240 million years to form a geode
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Geode Glossary
source: Dictionary of Geological Terms
| agate |
- a waxy variety of cryptocrystalline quartz (chalcedony) in which the
colours are in bands, clouds or distinct groups
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| agate nodule |
- a lense-shaped agate nodule
- fill holes can be seen & affect pattern of banding
- rapid deposition of groundwater causes dense agate to form
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| amygdaloid |
- volcanic rocks that contain numerous gas cavities filled with
secondary minerals such as zeolites, calcite, chalcedony or quartz
- filled cavities are called amygdules
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| amygdule |
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| aphanitic |
- pertaining to a texture of rocks in which the crystalline
constituents are too small to be distinguished with unaided eye
- used to describe igneous rock rhyolite
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| botryoidal |
- chalcedony forms bubbly surface
- found in the geodes of Kentucky & Indiana
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| chalcedony |
- cryptocrystalline quartz and much chert
- material agate is made of
- has a great array of colours: blue, gray, black, off-white, purple (deccomposition
of iron-bearing minerals)
- in Hall’s Gap geodes
- sulfide mineralization occurs in chalcedony geode
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| chert |
- a cryptocrystalline variety of quartz
- occurs as nodules, lenses or layers in limestone and shales
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| concretion |
- a nodular or irregular concentration of certain authigenic
constituents of sedimentary rocks and tuffs
- developed by the localized deposition of material from solution,
generally about a central nucleus
- are solid, grow from the center outward & are generally
noncrstalline though some crystals have been observed
- concretions are formed by the deposition of distinct minerals,
different from the surrounding rock, very firmly cemented around a
nucleus
- most common cementing materials are calcite, siderite & silica
- parts of plants, animals & well-preserved fossils may be found
at the nucleus
- harder than enclosing rock
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| enhydro |
- an agate with a visible bubble moving around when tilted
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| floaters |
- fully formed or doubly-terminated crystals that are unattached to
other crystals
- found in geode cavities & also in vugs such as "Herkimer
Diamonds"
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| geode |
- hollow, globular bodies
- subspherical shape
- clay film between geode wall & the enclosing limestone matrix
which is like a skin or crust
- an outer chalcedony layer
- an interior drusy lining of inward projecting crystals
- evidence of expansion or growth
- slow deposition of groundwater causes crystals to grow
- estimated it takes 240 million years to form
- geodes form either in sedimentary rock or in basalts/andesites
- hollows appeared in sedimentary rocks either because of animal
burrows, tree roots or mud balls
- word "geode" derived from Latin meaning
"earthlike", rounded shape
- found in USA: Montana, Indiana, Iowa, Illinois, Pennsylvania
(Lancaster Co. goethite geodes), Tennessee (Loretto), Oklahoma
(Comanche County etc.), Michigan (Houghton), Dugway geodes, Colorado
(Chaffee County ,Marshall Pass), Hauser Geode Beds, California
- amethyst geodes from Artigas, Uruguay or near Lajeado, Rio Grande Do
Sul, Brazil
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| industrial geodes |
- a.k.a. garden geodes - found in the garden
- solid all the way through, milky quartz
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| ironstones |
- large concretions of siderite & calcite found in shales
assoicated with coal beds in Kentucky
- name not much in use now
- collected just south of Holland, Michigan
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| Keokuk Geode |
- a specific type of geode which can be found in a 45 mile radius of
Keokuk, Iowa in the Tri-State area - Iowa, Illinois, Missouri
- geode is Iowa’s official state rock
- most geodes come from strata of the lower Warsaw Formation, a rock
unit of Mississippian age
- formed in sedimentary rock- shale, shaley dolomite & limestone
- original concretions thought to made of limestone or anhydrite which
then dissolved and was relaced by chalcedony for the skin and
interiors filled in over long periods of time
- source of mineralizing water is speculative as quartz is only weakly
soluble
- predominately quartz crystals
- silicates: crystaline quartz, kaolinite
- carbonates: calcite, aragonite, ferroan dolomite, malachite,
smithsonite, stilpnosiderite
- zinc sulfide
- sulphides: iron pyrite, sphalerite, chalcopyrite, millerite
- sulfates: barite, selenite, jarosite
- oxides: goethite, hematite, pyrolusite
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| lithophysae |
- hollow, bubblelike structures composed of concentric shells of
finely crystalline alkali feldspar, quartz and other minerals
- found in certain silicic volcanic rocks such as rhyolite and
obsidian
- a.k.a. thunder egg in the USA
- in France, come from the Esterel massif in the south-eastern part of
France
- found in Ploand
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| nodule |
- small more or less rounded body generally somewhat harder than the
enclosing sediment or rock matrix
- is solid
- agate, jasper & chalcedony form in nodules
- other common minerals that occur in nodules are siderite, gypsum,
calcite, quartz and barite/celestite
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| rhyolite |
- the aphanitic equivalent of a granite
- thunder eggs form in rhyolitic rocks - Oregon & southeastern
California
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| septarian nodule |
- septarian nodules that have a hollow cavity are sometimes known as
"septarian nodule geodes" or "geodes"
- Utah septarian nodules formed in ancient sea floor during the
Cretaceous period, 50 - 70 million years ago - filled with calcite -
yellow centres (calcite), brown lines (argonite), grey rock
(limestone), white or clear (barite)
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| septarium |
- a roughly spheroidal concretion, generally of limestone or
clay-ironstone
- cut into polyhedral blocks by radiating and intersecting cracks
which have been filled (and the blocks cemented together) by veins of
some material, generally calcite
- some contain fossils
- a.k.a. Septarian Nodule, Septarian Boulder, Turtle Stone
- presence of an "Avenue of Entry"
- in southern Illinois, they contain fluorite, sphalerite, witherite
& sometimes galena
- in Utah, the type has a different shape & may be filled with
agate
- thunder egg of Oregon is a septaria - cousin
- most geodes coming from France are in fact septaria (Drome, Ardeche,
Hautes-Alpes, Remuzat)
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| spherulite |
- a small radiating & usually concentrically arranged aggregation
of one or more minerals generally of spherical or spheroidal shape
- formed by the radial growth of acicular crystals in a rigid glass
about a common centre or inclusion
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| thunder egg |
- geodelike body commonly containing opal, agate or chalcedony
weathered out of welded tuff or lava
- form only in rhyolitic rocks, almost always in association with
perlite
- exterior surface is "warty" & it has a rind
- interior cross-section commonly exhibits a star-shaped outline
- interiors are filled with chalcedony, agate or opal
- when hollow, thunder eggs are sometimes referred to as geodes
- no thunder eggs exist in the USA east of Colorado except for a
single locality near Grand Marais on Minnesota’s North Shore
- is the common name for lithophysae
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| turtle stone |
- a septarium with distinctive external markings on the outside
resembling the shell of a turtle
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| vesicle |
- a small circular enclosed space
- a small cavity in an aphanitic or glassy igneous rock, formed by the
expansion of a bubble of gas or stem during the solidification of the
rock
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| vug |
- a cavity, often with a mineral lining of different composition from
that of the surrounding rock
- can form spherically, in veins, irregular openings
- vugs do not generally weather out of the host rock
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Geodes: Some Locations & Types
USA
- Indiana, Washington Co. near Salem Indiana
- clear calcite crystals on pink-orange dolomite
- thin walled
- clear calcite crystals on white quartz
- Iowa, Keokuk – Warsaw Strata
- Keokuk geodes are from a 45 mile radius
- borders Iowa, Illinois & Missouri
- botryoidal chalcedony with calcite cubes
- blue & green chalcedony
- pink calcite crystals
- sphalerite
- quartz iron – cheery orange, quartz oxide – cherry red
- dew drop diamond
- pink dog-tooth
- iridescent brown calcite
- industrial (garden bed) geodes – usually solid all the way through
- interior filled with milky coloured quartz
- found in people’s garden’s
- banded agate lining with crystals
- world-famous locality noted for its millerite in small geodes
- light pink amethyst
- Kentucky, South Fork Creek
- bumpy, brown round geodes in river bed
- stepped calcite, spaghetti, smoky quartz, botryoidal chalcedony
- orange-yellow solids, clear, white
- Sheffler Geode Mine – Keokuk Geodes
- gray, smoth skinned round
- clear quartz crystals
Brazil Rio Grande do Sul
- amethyst
- most fantastic geode known – 33 ft. in length, 16 ˝ ft. wide, 10 ft.
high
- estimated weight of seventy thousand pounds
- lined with purple amethyst
- a piece weighing 400 lbs. is in the Smithsonian Institute
Mexico, Chihuahua
- amethyst quartz, smoky, gray, white, lavender and brown quartz
- along with contrasting crystals of calcite, goethite
Uruguay
- adjoining Brazil, Rio Grande do Sol
Canada
- High Falls, Manitoulin Island, Ontario
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