You could try a quik setting compound such as Durabond(it is much harder then regular compounds), be sure you apply it smoothly and the tape is embeded correctly, if it cracks again, you may need to re-stabilize your foundation. Or check the the structure of the building where the crack is appearing.

Let me add, you may try re-fastening the rock on either sides of the crack before reparing it. I would suggest 1 1/4" drywall screws about 7" apart and sink the heads slightly beneath the surface of the rock.
This will only work if the crack is on a joint in the drywall and there is a joist or stud to screw into.

[This message has been edited by Chip (edited August 25, 2000).]