How to tell if a rock is a geode

Updated October 20, 2026. Based on the Iowa and Illinois geological surveys and Oregon’s geology agency; sources are at the end.

Only opening it is certain

A geode is usually a roughly round, lumpy rock with a shell of chalcedony, a fine-grained quartz. Because the shell is much harder than the limestone around it, geodes weather out and collect in stream beds in known geode areas. Hollow ones feel lighter than filled ones, but many geodes are solid inside, so the only sure test is to open it.

Shape: Roughly round, lumpy or cauliflower-like
Shell: Chalcedony, much harder than the limestone around it
Weight: Hollow ones feel lighter, but many geodes are solid
Place: Known geode areas, often weathered out into stream beds

Two geodes side by side: a round, lumpy gray rock and a broken one with a white shell lined with pale yellow crystals — Two geodes from Iowa: one whole, showing the lumpy outside, and one opened, lined with quartz crystals. Photo: Astynax, CC BY-SA 4.0.

Signs to check before you crack it

Sign	What to look for	How much it tells you
Shape	Roughly round, sometimes oblong or shaped like a fossil	Good hint
Outside	Lumpy or cauliflower-like shell	Good hint
Place	Weathered out of limestone, dolomite or shale in a known geode area, or in a stream bed there	Good hint
Weight	Hollow geodes are lighter than filled ones, but many geodes are solid	Weak hint
Hollow sound or rattle	No geological source we read supports it	Not a test

Sizes vary by region. Iowa geodes are typically 2 to 6 inches across, with rare ones up to 30 inches. Illinois geodes average 3 to 5 inches and range from under an inch to more than 2 feet.

What is inside

In the Iowa–Illinois geode area, quartz is the main mineral inside. Its crystals get larger and fewer toward the center and end in pointed pyramids. Calcite is common. Iowa’s survey has identified 17 other minerals, and the Illinois survey lists ones such as dolomite, gypsum, goethite and pyrite. Some geodes are hollow and lined with crystals; others are completely filled.

Geodes began as concretions in soft sediment. Their insides later dissolved, and groundwater deposited the minerals now lining them.

Simplified cross-sections, not to scale. Geode facts from the Iowa and Illinois geological surveys, thunderegg facts from Oregon’s geology agency. Diagram: Capyhound.

Geode or thunderegg?

Oregon’s state rock, the thunderegg, is labeled a geode by Oregon’s geology agency, but it forms differently. Thundereggs grow in volcanic welded tuff or perlitic rock, look plain outside, and hold colorful silica-rich material that shows when cut and polished. Midwest geodes form in limestone, dolomite or shale and are hollow or filled with crystals. See thunder eggs in Oregon.

A cut thunderegg with a rough brown rind around a cavity filled with blue-gray banded agate — A cut thunderegg from near Madras, Oregon: plain outside, agate inside. Photo: James St. John, CC BY 2.0.

Opening a geode

A sharp blow with a bricklayer’s or rock hammer is usually enough to crack an individual geode, according to Iowa’s geological survey.
Petroleum: some western Illinois geodes contain petroleum under enough pressure to squirt out when broken.
Our own advice, not from the sources: wear safety glasses, because chips fly.

Why a photo is not enough

The USGS says rocks and minerals must be examined in person from all sides for accurate identification, and are extremely difficult to identify from photographs. The same goes for an unopened geode: the outside can only hint at what is inside.

Where you may collect

In Iowa, most geode localities are on private land, so get permission first, and collecting is not permitted at Geode State Park. Oregon thundereggs can be collected at fee and free sites in central and southeastern Oregon. For public places in the West, see where to find geodes.

Keep a log of your finds

The Capyhound app will save each find with its photo, place and date, even with no signal.

Questions

How can I tell if a rock is a geode without breaking it?

Look for a roughly round, lumpy or cauliflower-like rock, and check where you are: geodes weather out of limestone, dolomite or shale in known geode areas and collect in stream beds. A hollow geode is lighter than a filled one, but many geodes are solid, so only opening it is certain.

Are all geodes hollow?

No. Some are hollow and lined with crystals; others are completely filled by crystals.

What is inside a geode?

In the Iowa–Illinois geode area, a chalcedony shell lined with quartz crystals, often with calcite and other minerals.

Is a thunderegg the same as a geode?

Oregon’s geology agency calls its state rock, the thunderegg, a geode. Thundereggs form in volcanic welded tuff or perlitic rock and hold colorful silica-rich material, while Midwest geodes form in limestone, dolomite or shale.

Does a geode sound hollow when you tap it?

Many websites say so, but none of the geological survey sources we read mention a sound test, so don’t rely on it.

How do I open a geode?

Iowa’s geological survey says a sharp blow with a bricklayer’s or rock hammer is usually enough to crack one open.

Where can I collect geodes?

In Iowa, most geode localities are on private land, so ask permission, and collecting is not allowed at Geode State Park. Oregon thundereggs can be collected at fee and free sites in central and southeastern Oregon.