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Egg fossil or just a pretty geode? Arizona find

Arizona Jon

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Found on Arizona mesa top.  What’s curious is the shell like structure that I have not seen in the many geodes I’ve found around here.  See 3rd and 4th pics for best outer shell view.  Any thoughts from the community?  Thanks






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I agree. Not an egg. Still, it’s a worthy contender for the “cool rock” shelf. :) 

The good thing about science is that it's true whether or not you believe in it.  -Neil deGrasse Tyson


Everyone you will ever meet knows something you don't. -Bill Nye (The Science Guy)

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Imagine a shell that thick on an egg. How would the hatchling get out? Jackhammer? That's your first clue that it's not an egg. It's a very interesting geode -- worthy of shelf space!





Fossil hunting is easy -- they don't run away when you shoot at them!

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Welcome to the Forum from Scottsdale.


It looks like chalcedony formed as an amygdule in a volcanic rock. Almond shaped cavities in volcanic rocks such as basalt are filled with many secondary minerals including quartz. Note the almond shape of the shell. I bet the mesa is a basalt flow. Where is the mesa? 


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As @DPS Ammonite stated, chalcedony. May be a geode or may be solid. I've found a ton of those near Lake Pleasant and south of I10 west of Tonopah. If the outer shell is solid enough,  when cut they can be beautiful when polished.

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