Sure would like to know where that rock came from, it is not one that I have ever seen around these parts.
If that rock was mine I would grind a small flat on it to look inside. Does that red run right through ?
If just an external coating, how did it survive weathering or creek tumbling?
IF it can be proven to be limestone, it might be a unique cave stone ( formed in a cave environment with
dripping water. )
Mother nature did some amazing things with mud before it became limestone.
If it's the same feature as Chris is reporting, he ought to smack a few of them open and see what the entire inside might look like. The "black" he notes might be weathered hemitite (iron) that can be a very dark rust / near black.
let me see if i have some..... i'm going to try and post this rock ..to me.. it looks like a petrified dog print.... maybe not petrified but a dog print in a rock~.. how bout that....let me see what i can find.