For rockhounds, teachers and mineral lovers!

Geodes and Thundereggs from around the world!

Geodes are hollow rocks with crystals grown inside them! Classic localities are found in Illinois, Iowa, Indiana and outside the US such as Mexico and Brazil. Most geodes are sedimentary, while thundareggs are volcanic. 

Thundareggs are solid balls of rock that have chalcedony agate and sometimes crystals in side. Thundareggs are formed very differently than geodes. 

37 products

Geodes and thundereggs are great to collect and find for all ages. Thundereggs and geodes can be cut and polished for display or for use in crafts and lapidary arts.  

What is the difference between Agates, Thunder Eggs and Geodes?

Agate can be formed in veins, pockets, vugs, stalagmites, stalactites, stalegmites, cathedrals, and also be part of thunder eggs and geodes. Many popular agates are found in volcanic areas of Brazil, Argentina and parts of the western United States. Lake Superior agates are believe to have been brought south by the glaciers. 

Vein agate      Brazilian Agate        Vein and Fortification Agate

Geodes can be formed in sedimentary rock such as limestones. As water percolates the surrounding rock, it mixes with trapped gases and picks up minerals solutions that dissolved out from the surrounding rocks. Over time, these minerals come out of solution and form a hard shell and then layers of silica gel or crystals.The crystals grow in the hollow. Over time, these nodules are eroded out of the soils and tumble  down rivers and creeks.

 Geode example 

A thunderegg is not a geode as they are usually solid, and filled with chalcedony or agate, in a distinctive star pattern. Thundereggs form under specific volcanic conditions, with just the right mix of liquids, heat and pressure. Thundereggs may have a hollow, or filled with agate, opal or other minerals. Found in Oregon, Idaho, Utah, California, Nevada and Arizona. The outer structure can reveal a thunderegg - raised ridges in the rhyolite is the first clue, a warty exterior is the second, and a geometric structure inside when opened. 

Thunder Egg Example 

When is it a Geode and not a Thunder Egg?

Popular theory has Thunder Eggs formed in gas pockets of volcanic lava material, where the gases from the lava get trapped, and these gasses mix with ground water that also has minerals. As the nodules cool under pressure, this contributes minerals to the silica solutions trapped inside.This can create all kinds of banding - fortification, tubes, moss type, spheres, waves and layers, each with distinctive colors and patterns. A thunder eggs is usually a solid nodule filled with agate, but sometimes may have a hollow where crystals have also formed. One characteristic of a thunder egg is a five-pointed star pattern in the interior. 

Thunder Egg Example            Geode example 

A geode has a hard exterior crust or rind, lines with chalcedony or agate, and the center may be hollow, filled with crystals. In the past 200 years, scientists have developed theories how geodes form. All involve silica and water solutions that interact with the environment inside the geode - whether gases or liquid silica-rich solutions-that precipitate solids that grow in to chalcedony or crystal structures. As new chemicals are introduced by water dissolving the surrounding matrix resulting in different mixes of chemicals over time, into the same geode, encouraging the growth of other crystals on top of the original crystals. 

or example, a solution with small amounts of iron would create purple amethyst crystals, while another salt or metal would affect both the color and crystal shape. So, you can have an outer layer of chalcedony, then quartz, then calcite or dolomite. Other minerals can also be found inside, such as millerite, sphalerite, and, many times, another layer of quartz.