Geodes

A geode is a round rock with a hollow or crystal-filled space inside. From the outside it looks plain and lumpy. Crack or cut it open, and you may find sparkling quartz crystals, calcite or bands of agate. Thunder eggs are close cousins that form in volcanic rock. This guide covers both.

Geode is not the name of a mineral. It names the whole rock: the outer shell plus whatever grew inside. So two geodes can hold very different minerals.

How to Identify a Geode

Look for these clues. No single one proves it, so use them together.

• Shape: roughly round. Many are lumpy, like the top of a cauliflower.
• Size: most Iowa geodes are 2 to 6 inches across. A few are much bigger.
• Outside: plain and rough. It gives little hint of what is inside.
• Inside: hollow, partly hollow or filled with crystals. Many geodes are solid inside.
• Weight: a hollow geode can feel light for its size. This is only a hint, because many geodes are solid.
• Where it lies: geodes gather in certain rock layers and in the streambeds below them. Where you find one, look for more.

How Geodes Form

Many geodes start as a gas bubble in cooling lava. Later, hot water seeps through the rock. It carries dissolved silica, the stuff quartz and agate are made of. The silica builds up on the walls of the bubble, layer by layer, over many rounds.

If the space fills up, the geode is solid. If it does not, the center stays hollow, and crystals point into the empty space. That is how the New Mexico Bureau of Geology explains the geodes and thunder eggs at Rockhound State Park.

Not every geode comes from lava. Most of Iowa's famous geodes come from one layer of rock, the lower Warsaw Formation. Their crystals are mostly quartz, often with calcite.

Thunder Eggs vs. Geodes

A thunder egg is a close cousin of the geode that forms in volcanic rock. Oregon's state geology agency (DOGAMI) even writes "thunderegg (geode)." Here is what sets thunder eggs apart.

• Where they form: in volcanic rock. In Oregon that means welded tuff (rock made of volcanic ash) and perlite (a kind of volcanic glass). In New Mexico it is rhyolite, a pale lava rock.
• Outside and inside: a dark outer shell and a lighter core. From outside, most look like any other round rock.
• What fills them: agate, chalcedony and quartz, laid down by hot water. The center can be hollow, partly hollow or solid.
• Size: from less than an inch to more than 4 feet across, DOGAMI says.
• How to see them: cut and polish one. That is when the colors and patterns show.

Oregon's Legislature named the thunder egg the state rock in 1965, after rockhounds voted for it. DOGAMI says most come from Crook, Jefferson, Malheur, Wasco and Wheeler counties. Our Oregon rockhounding guide covers a public thunder egg site in the Ochoco National Forest.

Where to Find Geodes and Thunder Eggs

Geodes come from both volcanic rock and layered rock. The famous Keokuk geodes come from within about 35 miles of Keokuk, Iowa. Iowa made the geode its state rock in 1967. Thunder eggs are best known from Oregon.

Verified places you can actually go

Each of these rests on a published rule from the agency or operator, checked in August 2026. Rules change, so confirm before you drive.

• Rockhound State Park, New Mexico. The rare park where taking rocks home is the point. The state rule allows up to 15 lbs per visitor, souvenirs only, hand tools only. Agate is common; thundereggs and geodes are the prized finds.
• Dugway Geode Beds, Utah. BLM ground in the west desert. Our Dugway guide covers access and what the beds produce.
• Wiley Well District, California. BLM-listed geode district south of I-10 near Blythe, with the Hauser beds as the classic stop. Graded dirt roads past the pavement, and a winter destination, not a summer one.
• The Keokuk region, Iowa–Missouri–Illinois.The famous tri-state geode country. Most productive ground is private or permission-based; the reliable route in is a fee dig such as Sheffler's on the Missouri side, by appointment. Our Keokuk region guide covers the access story honestly.
• White Fir Springs, Oregon. A Forest Service rockhounding site in the Ochoco National Forest with jasper-filled thunder eggs, for personal use. Stay out of the nearby Mill Creek Wilderness, where collecting is not allowed.

All of these sit on our rockhounding map with coordinates and the source behind each access claim.

What People Confuse With Geodes

The most common mistake is calling every round rock a geode. First check for a hollow center or a crystal lining.

Collecting and Opening Tips

• Learn the place first. Find out which rock layers and streambeds give up geodes before you dig.
• Don't trust weight alone. A light rock does not prove a geode. Some geodes are solid, and some are only partly hollow.
• Open them with care. One careless hit can crack a good center. For a clean, flat face, use a saw. Our lapidary saw guide explains which kind to use.
• Cut thunder eggs to see them. Their patterns show best when cut and polished.
• Check the rule for each site. Limits, tools and fees differ from place to place.