GeodeMohs 7
The general case: outer shell with a hollow cavity inside, lined with crystals pointing inward. Hand-feel is lighter than a solid rock of the same size.
Photo: Wikipedia contributors · wikipedia
Geode Identifier
A geode is a hollow rock cavity lined inwards with crystals. A nodule is the solid version of the same nodular shape. A thunderegg is a rhyolite-hosted cousin with a star-shaped agate core. People mix the three up constantly, and the photo identifier is built around telling them apart. Upload a cracked or uncracked specimen and you get three ranked candidates with the lining mineral, the host environment, and the closest look-alike to rule out.
- Tells geodes from thundereggs and septarian nodules apart
- Names the lining mineral (quartz, amethyst, calcite, celestine)
- Reads host rock when visible (limestone, basalt, rhyolite ash)
- Works on cracked specimens and uncracked nodules
Reviewed by RockHoundR Field Team · Field identification & geology editors · Last verified
Quick answer
Geodes are hollow; nodules are solid; thundereggs are rhyolite-hosted with a star-shaped agate core and usually solid; septarians are crack-filled concretions. Photograph the broken face if you have it. The identifier returns three ranked matches with the lining mineral (quartz, amethyst, calcite, celestine), host rock, and the look-alike to rule out.
A geode is a hollow cavity in a host rock, lined inwards with crystals that grew from mineral-rich fluids. The cavity itself usually started as a gas bubble in a lava flow, a fossil hollow in a marine limestone, or a soft-sediment void that the host rock formed around. As silica- or carbonate-rich groundwater later moved through the cavity, layer after layer of chalcedony or calcite built up on the walls, often capped by an inward-pointing crust of quartz, amethyst, or calcite crystals. The shell is what protects the lining and is what makes a geode a geode: an outer skin of dense, fine-grained material and a hollow interior full of crystal points.
The classic confusion is geode versus nodule versus thunderegg. A nodule is the solid version of the same nodular shape: same outside, no cavity inside, the whole interior filled with chalcedony, agate, or jasper. A thunderegg is a rhyolite-hosted nodule from the Pacific Northwest with a star-shaped agate core surrounded by an ash-rind, and most thundereggs are solid rather than hollow. A septarian nodule is a concretion shot through with mud-crack-style cracks filled by yellow calcite and brown aragonite, and it is solid throughout. The identifier reads the broken face when you have one and is honest that uncracked round rocks are hard to call from a photo alone, since the only proof of a hollow is the sound and weight in your hand.
Locality narrows everything. Keokuk-style geodes from the tri-state area of Iowa, Illinois, and Missouri are smallish chalcedony shells with quartz, calcite, dolomite, and sometimes pyrite or sphalerite inside, hosted in Mississippian limestone. Brazilian and Uruguayan amethyst geodes are huge basalt-vesicle cavities lined with deep purple amethyst points. Mexican coconut geodes (Las Choyas) are hollow chalcedony shells with fine quartz and selenite linings. Oregon and central Washington produce thundereggs in rhyolite ash beds. Use the locality field heavily, since the same crystal lining can mean different things in different host rocks. The identifier never invents locality details you did not provide.
Visual identification guide for geodes
Reference pieces for the most-found geodes and their solid cousins. Each card shows the single feature that places a specimen.
Amethyst GeodeMohs 7
Purple drusy quartz lining. Large basalt-hosted Brazilian and Uruguayan pieces are the iconic form. Color saturation is deepest at the tip of each point.
Photo: Nkansahrexford · wikimedia
Citrine GeodeMohs 7
Yellow-to-orange quartz lining. Most commercial citrine geodes are heat-treated amethyst; natural citrine has a softer, less reddish tone.
Photo: Mahdikarimi70 · wikimedia
Keokuk GeodeMohs 7
Small chalcedony shell, drusy quartz lining often with calcite, dolomite, or pyrite crystals. Tri-state Mississippian limestone classic.
Photo: Astynax · wikimedia
ThundereggsMohs 6.5-7
Rhyolite ash host with a star-shaped agate core. Usually solid rather than hollow. Oregon and Washington are the classic source.
Photo: Chris857 · wikimedia
Septarian NodulesMohs 3-5
Solid concretion shot through with mud-crack-style internal cracks filled with yellow calcite and brown aragonite. Not a geode, but constantly confused with one.
Photo: Kent G. Budge · wikimedia
Geode and look-alike comparison
Lining mineral and host rock decide most calls. Hardness on the lining and the acid reaction settle the rest.
| Specimen | Hardness (Mohs) | Luster | Habit | Field tell |
|---|---|---|---|---|
| Geode | 7 | vitreous | nodular | Hollow cavity with inward-pointing crystals. Lighter in hand than a solid rock the same size. |
| Amethyst Geode | 7 | vitreous | drusy crystals lining a cavity | Purple drusy quartz lining, deepest color at tip; Brazilian basalt host. |
| Keokuk Geode | 7 | vitreous | druzy | Chalcedony shell with mixed quartz/calcite/dolomite inside; Mississippian limestone host. |
| Thundereggs | 6.5-7 | vitreous | nodular | Star-shaped agate core in rhyolite ash. Usually solid. |
| Septarian Nodules | 3-5 | dull to vitreous | nodular | Solid; internal crack network filled with yellow calcite + brown aragonite. |
| Nodules | Varies | Varies | nodular | Solid throughout; chalcedony, agate, or jasper fill with no cavity. |
Common geodes and their cousins
Quartz Geode
The default. Drusy clear-to-milky quartz lining. Mohs 7, scratches glass.
Amethyst Geode
Purple quartz lining. Brazilian and Uruguayan basalt geodes are the famous large form.
Citrine Geode
Yellow-to-orange quartz lining. Often heat-treated amethyst sold as citrine; natural citrine geodes are uncommon.
Calcite Geode
Carbonate lining: scalenohedral 'dogtooth' or rhombohedral crystals. Mohs 3, fizzes in dilute acid.
Celestine Geode
Sky-blue tabular celestine crystals. Crystal Cave on South Bass Island, Ohio is the iconic locality.
Thunderegg
Rhyolite ash host, star-shaped agate core, usually solid. Pacific Northwest specialty.
Keokuk Geode
Mississippian limestone, chalcedony shell, quartz + calcite + dolomite + pyrite inside.
Septarian Nodule
Concretion with mud-crack-style internal cracks filled with yellow calcite and brown aragonite. Solid, not hollow.
Identify by lining mineral
The crystal lining decides the name. Read color, habit, and hardness.
- Clear / milky quartz
- Default 'Quartz Geode'. Mohs 7, vitreous, six-sided pyramidal points.
- Purple quartz
- Amethyst geode. Iron-induced color, deepest at the crystal tip.
- Yellow / orange quartz
- Citrine geode. Often heat-treated amethyst; natural citrine has softer yellow without the reddish tinge.
- Calcite
- Calcite geode. Scalenohedral 'dogtooth' or rhombohedral, Mohs 3, fizzes in dilute hydrochloric acid.
- Sky-blue tabular crystals
- Celestine geode. Mohs 3 to 3.5, the South Bass Island Ohio specialty.
- Pyrite or sphalerite
- Common minor mineral in Keokuk-style geodes, growing in with the quartz and calcite lining.
Geode vs nodule vs thunderegg vs septarian
The single most-asked distinction in this category. The shell, the host rock, and what's inside decide it.
- Geode
- Hollow cavity, crystal lining pointing inward. Outer shell, lighter in hand than a solid rock the same size.
- Nodule
- Solid version of the same shape. Chalcedony, agate, or jasper fills the whole interior. No cavity.
- Thunderegg
- Rhyolite ash host with a star-shaped agate core. Usually solid. Pacific Northwest specialty.
- Septarian Nodule
- Concretion shot through with mud-crack-style internal cracks filled with yellow calcite and brown aragonite. Solid throughout.
- Geode in basalt
- Vesicle-filled cavities lined with amethyst, calcite, or zeolites. Brazil, Uruguay, India are the major sources.
- Geode in limestone
- Fossil cavity or void filled with chalcedony and quartz. Keokuk-area Mississippian limestone is the classic example.
How the geode identifier works
- Step 1
Photograph the broken face
If the geode is cracked, the lining is the answer. If it's uncracked, show the shell and add a heft note in the location field. JPG, PNG, or WebP up to 6MB.
- Step 2
Add locality if you know it
Iowa Keokuk, Brazilian basalt, Oregon thunderegg country, Mexican coconut. Locality decides the host rock and weights candidates strongly.
- Step 3
Get 3 ranked matches
Each match: lining mineral, host rock, and the look-alike (thunderegg, septarian, nodule) to rule out.
Take a photo that identifies well
- Photograph the broken or cracked face. Lining minerals are everything.
- Show the outer shell color and texture, since that hints at host rock.
- Include a coin or hand for scale, especially for large amethyst pieces.
- Backlight a thin chalcedony shell to show banding under transmitted light.
What to avoid
- Photos of just the unbroken outside, since you can't tell hollow from solid by eye.
- Flash, which washes out crystal color zoning.
- Multiple unrelated pieces in one frame.
- Through-glass display-case shots.
How accurate is this geode identifier?
Photos of a cracked-open geode identify well, because the lining and host are visible. Uncracked nodules are harder, since the only proof of a hollow is the sound and weight in your hand.
Strong on
- Cracked geodes where the lining mineral and habit are visible.
- Telling thundereggs and septarians from true geodes when the broken face is in the photo.
- Naming the host rock context (basalt, rhyolite, limestone) from the outer shell color and texture.
Less reliable on
- Uncracked round nodules, where the only proof of a cavity is heft and sound.
- Natural citrine vs heat-treated amethyst when sold as citrine.
- Distinguishing pyrite from marcasite in mixed-mineral geode linings.
- Locality-only attributions (e.g. specific Mexican coconut-geode source).
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Geode Identifier FAQ
What's the difference between a geode and a nodule?
A geode is hollow; the inside of the shell is lined with crystals that grew inward from the walls. A nodule has the same outer shape but is solid all the way through, filled with chalcedony, agate, or jasper instead of an open cavity. You can sometimes tell from heft (a geode is lighter in hand than a solid rock the same size) and from a hollow tap, but the only certain proof is to crack it.
What is a thunderegg and is it the same as a geode?
A thunderegg is a nodule that grew in a rhyolite ash bed, with a star-shaped agate core surrounded by a rind of weathered ash. Most thundereggs are solid rather than hollow, so they are technically nodules, not geodes. The Pacific Northwest (Oregon, Washington) is the world's richest source. Thundereggs were named the official state rock of Oregon in 1965.
Are amethyst geodes naturally that purple?
The deep purple of Brazilian and Uruguayan amethyst geodes is natural, caused by iron impurities exposed to natural radiation during the basalt's cooling history. Some pieces are color-enhanced or dyed in low-cost commercial channels, but the classic mine-fresh geodes show natural color zoning, deepest at the crystal tip and fading toward the matrix.
Is a citrine geode usually heat-treated amethyst?
Yes, most often. Natural citrine geodes are uncommon and command a premium; the bulk of bright orange-yellow 'citrine geodes' on the market are amethyst geodes that have been heated to convert the iron color centers to yellow. Both are real quartz, both look fine, but the price and rarity differ. A reddish-orange tone and a flat color zoning hint at heat treatment; a soft, gradient yellow hints at natural.
How do I crack a geode without smashing the crystals?
Score the equator with a hacksaw or rock saw, then place it in a sock or rag and tap firmly along the score line with a hammer. A geode cracker (a hinged steel device) gives the cleanest break. Avoid using a chisel or just hammering, which usually destroys the crystal interior. Wear safety glasses since the chalcedony shell shatters in sharp fragments.
Where are geodes found in the United States?
The Keokuk area at the corner of Iowa, Illinois, and Missouri is the iconic U.S. geode locality, with limestone-hosted chalcedony shells lined with quartz, calcite, and pyrite. Indiana, Kentucky, and Tennessee produce similar limestone geodes. The Pacific Northwest produces thundereggs. Mexico's Las Choyas and Brazilian basalt fields produce the imported coconut and amethyst geodes you see in shops.
Is this geode identifier free?
Yes. Three free identifications per day per device, no signup, no install. The RockHoundR app removes the daily limit, saves every crack with photos and notes, works offline at the dig, and overlays results onto 250,000+ rockhounding spots with geology and land-access data.
References & sources
Property data and reference imagery used on this page are cross-checked against the following sources.
- Iowa Geological Survey: Iowa Geodes
State survey reference on geode formation, lining minerals, and the Keokuk region. Geode is Iowa's state rock.
- Mindat.org: Geodes
Reference article on geode host rocks and the minerals that line them.
- Mindat.org: Thunder Egg
Reference for the rhyolite-hosted thunderegg type and the geode vs thunderegg distinction.