My dad found this as he was land surveying up in the high deserts and picked up this weird what ever it is and gave it to me. I have been wondering what it is?
What happen long ago, was that different moist sediments joined together in layers while gathering a pocket of air that was trapped inside. Over many years the sediments hardened and was eroded out and broke open showing the different layers and also the void within. Hope this explains what you have there.
Sedimentary rock, likely sand stone and as others have chimed a concretion. old digger gives a good example of one type of concretion another type that is more common forms similarly however the center is usually solid, or if hollow has erroded out of the formation. Indians would find large concretions that have broken in half and hollowed to use a makeshift bowls or paint pots.
Under higher magnification the grey ring seems to have a gemmy quality. Almost a smoky quartz look. Resembles an interrupted forming of a geode. Or my magnifier is broken.