What is this? Fossilized Acorn?

[Flutterby84]

Hello Everyone! 

I have found some of these while Rockhounding in Rio Rancho, NM as well as near Nageezi, NM areas. I THINK they are possibly Acorn tops, or possibly even an Oak seed? But, I am not for certain! It is driving me NUTS not knowing what they are. My boyfriend thinks they may possibly be reminisce of shell casings, as he found some nearby... I am almost certain they are not she'll casings. But, When I have found them, they aren't all over, rather specific areas. I have found them as small as a BB all the way up to the size of a small dinner plate in diameter. And whenever I've found them, they aren't whole. They seem to always have the same similarities of the ones I've found in the past. They always seem to be hollow, though full of sandstone in the center/core... Some seem to be almost perfectly round, and others more like the Acorn shape we know today...

Please, if anyone can tell me for certain what they may be, I would be so grateful. I am not needing for any other reason than my own knowledge and the fact that it is driving me nuts not knowing for certain! Thanks in advance for your input!  

 

 

[Top Trilo]

These appear to be concretions and on some of them the softer interior eroded, leaving just the outer layers.

[DPS Ammonite]

They are Moqui Marbles, AKA iron-rich concretions.

 

https://geology.utah.gov/map-pub/survey-notes/glad-you-asked/moqui-marbles/

[Coco]

For your future photos, use a ruler in inch or cm, or click on my last link in my signature, print the document and place your samples on it

 

Coco

[Flutterby84]

DPS Ammonite, That is exactly what they are!! Thank you so much for the knowledge! Much appreciated!!