The Heavenly Stone is a quest item found in Diablo I needed for the Magic Rock quest. This quest is initiated by Griswold, who says that the stone was stolen from a caravan by some masked thieves. It is found on a wooden pedestal on Level 5. The character will say: "This must be what Griswold wanted", after retrieving it.
Various descriptions of the stone suggest that it is actually a meteorite that fell somewhere near Tristram.