The Eye of Tiamat is a unique gem which has a small chance to drop after defeating the Ancient Guardian in one of the maps that are sold in Mapworks.
"I was farming Lv56~65 maps and found a Lv60 Ancient Guardian. It supposedly has 5% chance to drop but I had to kill him 54 times before getting my first drop. Using Sweet Aide exploit, you can save & load to repeatedly kill a boss until you get the drop you want."