It has an Item Level of 1 and requires a character level of 1.
"It seems to be active but it's difficult to tell."
There is only one of two possible dungeons in the Fields of Slaughter at a time, one being the Icefall Caves, the other being Caverns of Frost. The caves can spawn in any of several possible locations on the map.
Ballista spawn points share the same locations as the Caverns of Frost/Icefall Caves. If Machines of War has been completed, the ballista spawn points are still a possibility; they will just be empty. At least one of the two possible dungeons will always be on the map regardless what quest is currently active.
Also note that after the checkpoint after completing Machines of War, Tyrael will join you for a short time. If you exit and restart to recreate the Fields of Slaughter map Tyrael will not immediately rejoin your party. When you move on to the Rakkis Crossing bridge Tyrael will reappear in your party.
When you find the Caverns of Frost you will be looking for a unique Lacuni Huntress called Chiltara, that usually spawns in the second level of the cave. Chiltara does not always spawn, so you may need to clear the dungeon several times. When she spawns, she has a rare chance to drop the Gibbering Gemstone. The item will be account bound as soon as it drops.
Gemstone drop rate is approximately 5%, but as of patch 2.0.5 the spawnrate of Chiltara has been increased.
- The gem itself is a reference to the hysteria-inducing "Chat Gem" that was present in the Battle.Net lobby in Diablo II. The gem was initially blue, but would turn purple and state that the gem had been "activated" in the chat window above. If clicked enough times rapidly the prompt would instead say, "Moo." It had no other noticeable effect, leading to rampant fan speculation. A similar gem is located at the bottom of the Diablo III forums.