Possibly. I'm not sure we have the words to frame what it is, but it's not a rock. The rock exists, but the Singularity itself is something else. If it is a brain, the rock might be its body, a form it is placed in.

Istredd has thought from the start it was more than a physical object, because he is used to monoliths. He knows the difference between him always communing with actual rocks vs. something humming at him from the Horizon. He thinks there may be more conclusions he can come to later, once he has time to consider. He doesn't manage to sit up completely, but he is able to lean on an arm, halfway up.

It didn't like foreign magic. I tried at one point to use my telempathy on it, and it made it hurt more. Lucifer probably felt it intensify at one point. I don't know if that's just a defense mechanism since we weren't supposed to be there.