What’s the Difference Between Thunder Eggs and Geodes?

Thunder eggs and geodes are sought-after by collectors and novices alike. Both are round and reveal beautiful interiors, however, there are some distinct differences between these two types of specimens.

Thunder eggs, which are technically known as lithophysae are typically found with solid or near-solid cores of chalcedony or agate, whereas geodes are relatively hollow inside with thinner layers of quartz crystals. 

This thunder egg from Haida Gwaii, called Star Spirit, has a solid interior of blue and grey chalcedony. 

This thunder egg from Haida Gwaii, called Star Spirit, has a solid interior of blue and grey chalcedony. 

This smaller geode has a quartz crystal centre with a small amount of amethyst colouring. Blue banded agate surrounds the crystal interior. This specimen is from Haida Gwaii. 

This smaller geode has a quartz crystal centre with a small amount of amethyst colouring. Blue banded agate surrounds the crystal interior. This specimen is from Haida Gwaii. 

This geode from Haida Gwaii has an empty core and its inner layer is covered with small amethyst crystals. 

This geode from Haida Gwaii has an empty core and its inner layer is covered with small amethyst crystals. 

These crystal clear Fairy Geodes were cut at Crystal Cabin in Haida Gwaii. 

These crystal clear Fairy Geodes were cut at Crystal Cabin in Haida Gwaii. 

Thunder Egg Formation

Thunder eggs are spherical concretions of solid or near-solid nodules filled with chalcedony, agate or quartz crystals. Crystal Cabin's Star Spirit Thunder Eggs from Haida Gwaii are formed in ash beds in the top sections of silica-rich rhyolitic lava flows. Generally, the formation of thunder eggs seems to be more common in the gas-charged ignimbrite eruptions.

Star Spirit Thunder Egg from Haida Gwaii

Star Spirit Thunder Egg from Haida Gwaii

The exterior of a Star Spirit Thunder Egg. 

The exterior of a Star Spirit Thunder Egg. 

As volcanic lava cools, a shell forms within the rhyolite ignimbrite around trapped gas bubbles. Flow-lines found throughout the ignimbrite are preserved in the shell. Compaction over millions of years causes these gas bubbles to eventually crack the rhyolitic shell and allows the gas or liquid to escape. This leaves behind a hollow cavity. Heated ground water leaches silica and other impurities from volcanic ash and deposits them into the cavity. The watery solution slowly turns to gel, but first creates a silica layer in the innermost core. The gel eventually hardens to deposit chalcedony, agate, quartz crystals, or a combination of these minerals. These minerals form the core of a thunder egg.