This is what happens when you are using mods during experimental testing. Something changed your block mapping ID's, and so now all the rocks are unknown.
Definitely mods, looks like something to do with nest. Sylen is correct, you could also check your output logs, search for "ERR" then you can see what's happening.
also happens when person is not on current build, playing on and older build, update to newest experimental should fix it.
in his screenshot upper right hand corner he is on B285, should have been on B289. Currently as of tonight B292