To see how this image of geode has been used in a video about fractals, check out http://youtu.be/93akxnQ1xxw

Blue Celestite Geode
MADAGASCAR

Dolomite "geode", Dolomite, Strontianite, Marcasite
Manitoulin Island, Ontario, CANADA

The hollow spherical geode section is partially filled in on the inside with smooth, beige, small botryoidal chalcedony. It looks like rounded gray peas.

Formation theory #1 - Brazil

• 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

• quartz is the most common material found in geodes

• colour depends on what impurities were picked up by the solutions

• if the solution is deposited slowly & has relatively few impurities, it results in crystal lined cavities

• if solution is deposited rapidly, cavity is filled with bands of fine-grained quartz - namely agate

Formation theory #2

• they were originally nodules of limestone or anhydrite

• formed within a soft sediment by concentric outward growth around small nucleus or core

• the interior of the concretion washed out, causing a hollow

• groundwater solutions then left behind a replacement of the geode walls & the crystals inside

• takes 240 million years to form a geode

• a waxy variety of cryptocrystalline quartz (chalcedony) in which the colours are in bands, clouds or distinct groups

• a lense-shaped agate nodule
• fill holes can be seen & affect pattern of banding
• rapid deposition of groundwater causes dense agate to form

• volcanic rocks that contain numerous gas cavities filled with secondary minerals such as zeolites, calcite, chalcedony or quartz
• filled cavities are called amygdules

• an agate pebble

• 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

• chalcedony forms bubbly surface
• found in the geodes of Kentucky & Indiana

• 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

• a cryptocrystalline variety of quartz
• occurs as nodules, lenses or layers in limestone and shales

• 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

• an agate with a visible bubble moving around when tilted

• fully formed or doubly-terminated crystals that are unattached to other crystals
• found in geode cavities & also in vugs such as "Herkimer Diamonds"

• 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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• a.k.a. garden geodes - found in the garden
• solid all the way through, milky quartz

• 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

• 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

• 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

• 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

• the aphanitic equivalent of a granite
• thunder eggs form in rhyolitic rocks - Oregon & southeastern California

• 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)

• 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)

• 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

• 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

• a septarium with distinctive external markings on the outside resembling the shell of a turtle

• 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

• 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