One of my hobbies back in my college years was cave-exploreing. There was even a club for that at the college. Most "natural" caves occur in Limestone geological areas. While hiking to a cave I'd often see unusually-shaped slabs of limestone, and when I asked about them, I was told (by a Geology Professor) that due to its Calcium chemistry, limestone often erodes/dissolves into peculiar shapes. (Notably, looking like partially-melted ice, with tunnels and ROUND-ISH HOLES in it.) So, based on my education as a Geology student and my personal experiences with encountering limestone rocks, I agree with AlabamaRelic that Sallysue's mysterious stone is a naturally-occurring slab of "peculiarly eroded" limestone.
Also, please note that Sallysue is in Texas, which has vast areas of eroded limestone rocks exposed on the surface of the ground.