The Quarry is located at 1419W 1099S, on the eastern slope of the Stonewall Range. It is marked on your map if you accept the quest from Edwin Wallows of Vooriden to fix the altar in the temple of Halcyon.
The entrance to Torin Quarry is a bit to the east (Point 1). The area is home to several monsters, the quest stone (Point 3), and the body of a dead hero (Point 2) which can be looted.
The stone is 8x8 and weighs 1000 pounds. It is the only available stone in the quest, so do not pick it up unless you plan on storing it, or plan on heading back to Vooriden to finish the quest.
Note that the stone can also be destroyed, so use caution about using explosives if fighting the nearby beasts. Loss of the stone will permanently botch not only the altar quest, but also a quest for Stringy Pete.