That’s a slab of lithographic limestone, with the image still on it. Normally, once the edition was complete, the image would be effaced from the stone, and used for other images. The same slab can (or rather could) be used over and over.
The limestone used in stone lithography comes from a quarry that is played out now. There is no more, no similar deposits have never been found, and the slabs in existence have become very thin indeed. Most lithographs today are printed from treated zinc plates.
This is not to be confused with offset lithography, which is a high-speed printing process involving the dots you referenced.
The object could be worth something. It might be worth something just for the limestone itself, which an artist could print from for decades after the effacing the image.
awesome, thats amazing you knew all of this. Thank you.