Curious Collections: True Blue Fossils

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Vertebrae fragments of Champsosaurus sp. Photo: ©AMNH



“The overwhelming majority of vertebrate fossils fall into the earth tones, browns, black, and white,” says Carl Mehling, collections manager for fossil amphibians, reptiles, and birds at the Museum. (Click here for a look at some of the biggest fossils in the Museum’s vertebrate paleontology collection, or sign up for a Members-only behind-the-scenes tour with Carl Mehling this fall.)

Specimens that deviate from this drab palette are rare, which makes finding them all the more exciting. Mehling, who discovered the fossils in the vast collections, couldn’t believe the color was natural. “I thought, ‘Get out of here!’” says Mehling. “I thought somebody had put a coating on it that just happened to be blue.”

The muted sapphire tone was no coating but rather a part of the bones, which had undergone a process called mineralization after burial. Dissolved minerals in groundwater deposited minerals in the bones, changing their chemical composition. What minerals could possibly turn a bone blue? Without extensive lab work, it’s hard to be sure.

“Typically, it only takes a very little amount of a mineral to change the color [of an object],” says George Harlow, curator of minerals and gems in the Department of Earth and Planetary Sciences.

But, so long as the structure of the bones is intact, what mineral caused this metamorphosis would be just a secondary concern to those who study fossils.
