You could spend a lifetime looking through fossils in the collection rooms of the Smithsonian Museum in Washington D.C. There are more than 40 million of them there, many collected by famous fossil hunters of centuries past, and many representing rare, unusual or even one-of-a-kind specimens.

And if you look long enough, you're bound to find something completely unexpected. Case in point: two fossilised crocodilian eggs, filled not with ancient embryos, but with large crystals, just like geodes.

This 49-million-year-old croc egg had its insides replaced with quartz and calcite crystals. Image: A. Telfer, courtesy of Smithsonian Institution

These incredible eggs were discovered in Wyoming in 1930 by George F. Sternberg of the famous fossil-hunting Sternberg family. They date back to the Eocene Epoch, about 49 million years ago. At that time, Wyoming was home to all sorts of creatures, including large mammals, small reptiles and an assortment of prehistoric crocodilians, from familiar-looking aquatic gators to bizarre hoof-toed crocs called pristichampsids.

"The eggs were studied in detail by researchers in 1992," Hans-Dieter Sues, Smithsonian Paleontology Department Chairman, told me in an email. "Based on the microstructure of the shell, this study confirmed that they were laid by crocodilians."

Prehistoric croc eggs are quite rare because they don't fossilise easily. They're generally small with very thin shells, and aren't particularly sturdy compared to the tough shells of birds. The eggs from Wyoming are less than 7cm long, with shells only about 0.7mm thick – this was a case of very lucky fossilisation!

So how did these rare, thin-shelled eggs end up full of crystals? It turns out they formed much the same way as normal rock geodes: with empty space and a lot of time.

Both "egg geodes" side by side. Image: Smithsonian Institution.

The formation of a geode begins with the creation of an open space inside a rock. This could be an air bubble left behind inside cooling lava, or perhaps a rock that formed around an animal or plant that later decomposed away. Whatever the case, as water percolates through the rock and into the empty hole, it carries minerals with it, slowly depositing them inside. Given enough time, those microscopic mineral deposits will build into big, beautiful crystals.

It may take thousands of years for a small cavity to fill with glittery contents. In a large space, like a cavern, the process can happen over the course of many millions of years, ultimately creating astonishing and enormous crystal formations.

The same thing happened to these eggs. "After burial, the interior of each egg decomposed, and the resulting void was filled in by calcite and quartz crystals, creating a geode-like structure," Sues explained.

The unhatched eggs provided nice enclosed spaces, and the porous eggshells would have allowed lots of mineral-rich water to seep through. Then it was just a matter of waiting a few millennia and voila! Egg geodes! 


Top header image: A. Telfer/Smithsonian Institution