Bone vs. Stone: How to Tell the Difference

There is no single hard-and-fast rule for distinguishing rock from bone, but there are a few principles that can definitely help you tell the difference

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When I was a child, one of my uncles gave me what he said was a real dinosaur bone. The little black object certainly looked like some sort of bone, and I kept it in my little collection of shark teeth and other fossils in my closest. After a while I almost completely forgot about it, but when I took a college course on dinosaurs I remembered the little thing. I took it to my professor to ask what kind of animal it might have come from.

It was not a fossil at all, my professor told me. The "dinosaur bone" was really a concretion, or a small lump of mineral that had formed around some bit of detritus. A broken part of the object made the identification easy. The exposed internal structure was compact, uniform, and smooth. It entirely lacked any sign of internal bone structure that a real dinosaur bone would exhibit.

Paleontologists respond to dozens of similar queries each year. Many people find concretions or vaguely bone-shaped rocks and bring them in to ask what kind of dinosaur the "bones" came from and if the museum would be interested in buying them. Needless to say, most of those people leave a bit disappointed that they have not uncovered the find of the century in their backyard, but these common experiences bring up a simple question: how can you tell fossil bone from stone?

There is no single hard-and-fast rule for distinguishing rock from bone, but there are a few principles that can definitely help you tell the difference. One of the simplest is that you need to know where to look for fossils. If you spot a "dinosaur egg" in the soil while mowing your lawn the chances are pretty good that is is just a rock. Real fossils will be found in particular rock formations which geological maps and even some state-specific booklets can help you identify. Before you grab your pick and shovel, though, you will have to familiarize yourself with the type of land those deposits are on and what the rules are about collecting fossils. If you just walk to a formation and pick out a fossil without filling out the right paperwork and being absolutely certain of where you are, you are probably breaking the law (not to mention the fact that trained paleontologists are much better qualified at properly documenting and excavating fossil sites).

But let's assume that, regardless of how it was acquired, you have what you think is a piece of fossil bone. Out of its geologic context it is impossible to compare it to the surrounding rock (fossils are often different in color and smoother than rocks from the same deposit), but if there is a break on the specimen you may be able to check its internal structure. A rock or concretion, like the one I showed to my professor, will be solid, and the inside of the rock will look like the outside. Fossil bone, on the other hand, will probably preserve the internal bone structure. In a fossil bone you will be able to see the different canals and webbed structure of the bone, sure signs that the object was of biological origin. You can even try a tongue test. The porous nature of some fossil bones will cause it to slightly stick to your tongue if you lick it, though you might want to have a glass of water handy if you feel compelled to try this.

By following these guidelines it becomes easier to determine whether or not you have really found a fossil bone. It does not take a Ph.D. education; just some attention to detail and common sense.

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