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I found this on a hike in Terlingua, Texas, near Big Bend National Park. It can't be scratched on the outside shell, but the white part on the inside is scratch-able with a knife. It is heavy.

First Side Second Side End

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closed as off-topic by user12525, uhoh, arkaia, Erik, Peter Jansson Aug 30 at 9:06

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This appears to be an irregularly shaped concretion. According to the National Geologic Map Database there is a Geologic Map Unit named Terlingua which contains some thin layers of concretionary limestone. This map from the same publication shows the rough extent of the appearance of this rock unit.

enter image description here

More information about concretions can be found on this wikipedia page where they are defined as:

A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil.

I doubt your rock is a septarian concretion, as shown in the picture below, but I'm having a hard time finding any other images of concretions similar to your rock.

enter image description here

Source: http://www.everythingselectric.com/septarian-concretions-mystery-pockets-quartz/

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