I mean before they are cracked or broken…
Is it a vacuum?
Is it air? (Perhaps from the time it was formed?)
Is it a gas left behind after the crystallization of the minerals? (most likely)
I’ve been unable to find references to anyone who has tried to find out.
Here is a site http://www.desertusa.com/magjan98/jan_pap/du_rock_geode.html
which explains the forming of the rock. Its formed by water precipitating the crystals which kinda disqualifies the vacuum theory.
I’ve cracked some and found water but around here with a frost level of 4 feet it may be seep water, after the original water has frozen,cracked the rock leaked out and been replaced.
Also any crack would have allowed the original content,gas or liquid,air is gas,to escape.