Marbles of Suzhou.

I wonder whether there is anyone who would be able to tell me what this is because I have no idea. I saw them in Suzhou in China in traditional pavilions in the famous gardens of the city. They are carefully framed and look as if they were pictures but they are not. It seems that these are cross-sections of marbles, framed and exhibited, sometimes next to a picture. I wander why. Is there a Chinese tradition of exhibiting marbles like that? I guess there must by, but where does it come from? Obviously there is no equivalent tradition in the Western world which is why I haven't found anything of the subject in a Western language.