Types of Geodes: The Main Kinds and Where They Form

Every geode belongs to one of two families, and knowing which one you have narrows the possibilities immediately.

Sedimentary geodes grow in limestone and shale. The Iowa Geological Survey describes the classic sequence: a mud concretion forms around organic material, its outer shell is replaced by chalcedony, the inside dissolves away, and quartz crystals grow into the cavity that is left. These geodes have a bumpy grey-white chalcedony rind and come out of creek gravels and quarry faces.

Volcanic geodes grow in lava. Gas bubbles trapped in cooling rock leave cavities that groundwater later fills. At the Dugway beds in Utah the host rock is rhyolite; in Brazil and Uruguay it is basalt, which is where the big amethyst cathedrals come from.

The host rock also predicts the fill. Sedimentary geodes give you quartz and calcite with a wide supporting cast; volcanic ones lean towards quartz, amethyst and agate.

• Keokuk quartz geodes (Iowa, Illinois, Missouri). From the lower Warsaw Formation of Mississippian age, concentrated within about 35 miles of Keokuk. Typically two to six inches, rarely up to 30. Quartz dominant, calcite common, 17 further minerals recorded. Iowa's state rock since 1967.
• Kentucky geodes. From the Fort Payne and Warsaw-Salem formations, most under a foot across with some reaching two feet, and known for variety: dolomite, barite, galena, fluorite, pyrite, sphalerite, selenite and celestite all occur alongside quartz and calcite.
• Brazilian and Uruguayan amethyst geodes. Basalt-hosted, cut open and stood upright as cathedrals, from centimetre pieces to specimens over three metres tall.
• Las Choyas coconut geodes (Chihuahua, Mexico). Mined from rhyolitic ash tuff more than 125 feet down; typically 2.5 to 3.5 inches, occasionally six; eighteen minerals recorded, quartz and calcite most common. About 80% come out solid.
• Dugway geodes (Utah). Rhyolite-hosted, two to four inches, usually clear, purple or pink quartz, on Bureau of Land Management public land.
• Thundereggs (Oregon). Agate-filled spheres from rhyolite lava flows, the state rock since 1965, found chiefly in Crook, Jefferson, Malheur, Wasco and Wheeler counties.
• Celestite geodes (Ohio, Madagascar). Pale blue strontium sulfate crystals. The Crystal Cave at Put-in-Bay, Ohio is the famous one.
• Agatized coral geodes (Florida). Silica replaced the coral skeleton, leaving drusy quartz pockets; Florida's state stone since 1979.
• Septarian nodules (Utah). Cracked concretions with calcite-filled veins rather than an open cavity, so not true geodes.

Work from the outside in.

• Rind first. A bumpy grey-white chalcedony crust points to a sedimentary geode from limestone country. A dark grey, hard, smooth sphere points to volcanic rock.
• Then colour of the fill. Purple in a thick basalt shell means the Brazil-Uruguay trade. Pale sky blue blades mean celestite. Banded agate with no open cavity means a thunderegg.
• Then size. Under an inch, split in a display box: a Tabasco geode. Two to four inches, dark grey, from Utah: Dugway. Two to six inches with a lumpy white shell from a Midwestern creek: Keokuk.
• Then the giveaway details. A green glow under shortwave ultraviolet suggests Trancas material. A fossil coral texture on the outside means Florida agatized coral.

If the inside shows radiating cracks filled with yellow calcite rather than an open cavity, you have a septarian nodule, which is a concretion rather than a geode.

Rarity in geodes is mostly about the cavity, not the mineral. Quartz-lined geodes are abundant: Iowa alone has supplied collectors for generations. What is genuinely uncommon:

• An intact, undamaged cavity in a large piece. Most geodes are completely filled with minerals in the first place, as the Kentucky Geological Survey notes, so a big clean open cavity is the scarce thing.
• Unusual minerals in place. Celestite, fluorite, barite and sphalerite linings are far less common than quartz and calcite.
• Deep even colour. Amethyst that holds strong purple across every crystal tip is the top of the Brazilian and Uruguayan trade.
• Documented locality. A geode with a named mine and date attached is worth more than the same rock without one, and the label costs nothing to keep.

Geode guides often sort geodes by what grew in the cavity rather than where they came from. Geology In's list adds these to the families above:

• Calcite geodes. Sedimentary, with blocky or dog-tooth calcite. Soft at hardness 3; a copper coin scratches the crystals.
• Celestine geodes. Pale blue strontium sulfate, as at Crystal Cave, Ohio.
• Pyrite geodes. Brassy yellow iron sulfide crystals. The Kentucky Geological Survey lists pyrite among the minerals found with calcite and dolomite in Kentucky geodes.
• Chalcedony and botryoidal geodes. Rounded, grape-like linings with no pointed crystals; see our chalcedony geode page.
• Drusy geodes. A carpet of tiny sparkling crystals rather than large points.
• Enhydro geodes. Rare pieces that still hold trapped water inside.
• Pseudomorphs. One mineral that has replaced another and kept its shape, such as Florida's coral geodes.

The list of possible linings is long. The Kentucky Geological Survey names quartz, calcite and dolomite first, then bitumen, barite, galena, fluorite, limonite, sphalerite, pyrite, selenite and celestite, and the Iowa Geological Survey records 17 minerals beyond quartz and calcite in Keokuk geodes.

No. Agate and geodes form the same way but finish differently. The Minnesota Department of Natural Resources describes Lake Superior agates as forming when water seeped into the holes left by gas bubbles in lava and deposited quartz and other minerals in layers. If the layers fill the hole to the centre, the result is a solid agate nodule.

A geode keeps an open cavity. The Kentucky Geological Survey defines geodes as hollow, with crystals growing inward from an outer shell, and FossilEra puts the distinction plainly: geodes are hollow cavities, while nodules are completely filled in with minerals. The Iowa Geological Survey adds that many geodes are solid, filled by crystal growth.

So an agate can be the shell of a geode, as in an agate geode, but a solid agate is a nodule, not a geode.

Two hopes come up again and again with rare geodes, and the published mineral lists answer both.

• Diamonds. Neither the Kentucky nor the Iowa survey lists diamond among geode minerals. The clear, sparkling crystals sold as Herkimer diamonds are, in Wikipedia's words, double-terminated quartz crystals and not diamonds; they come from cavities in dolomite in and around Herkimer County, New York.
• Gold. Gold is not on the Kentucky list either. What is on it is pyrite, the brassy iron sulfide often called fool's gold, along with galena and sphalerite. A gold-coloured metallic crystal in a geode is far more likely pyrite than gold.

What actually makes a geode rare and worth more is covered in the section above on which types are rare: an intact large cavity, an uncommon lining such as celestite, fluorite or barite, deep natural colour, and a documented locality.

For prices, FossilEra's geode section listed 43 pieces from US$5 to US$29,000 on 25 September 2026, and Paxton Gate asked US$1,475 for a 10-inch black botryoidal chalcedony geode with calcite.

Four questions, in order, identify most geodes:

• Is it hollow? If it is cut, look for an open cavity. If not, the Iowa Geological Survey's rule applies: a hollow geode is noticeably lighter than a solid nodule of the same size.
• What is the rind? A bumpy chalcedony crust points to limestone country; a dark, smooth sphere to volcanic rock; a coral pattern to Florida agatized coral.
• What are the crystals? Pointed six-sided crystals that scratch glass are quartz or amethyst. Blocky crystals a copper coin scratches are calcite. Pale blue blades are celestite. Rounded waxy bumps are chalcedony.
• Where did it come from? Match it to the locality list above.

A photo of the cut face in daylight with a coin for scale is enough for RockLens to suggest a likely type; confirm it with the scratch test before you label it.

The sections added on 25 September 2026 were researched and drafted with AI assistance and checked line by line against the sources listed on this page. Prices are dealer asking prices seen on that date, not appraisals.