Underground Outpost
Find this location in Klysma Quarry, near your entry point to Sinai. You'll pass through here during the Rise of Shaqilat quest.
Use the rope stretched above the quarry entrance to dropkill the outer guard (the inner guards can't see this far).
There are three guards inside. You can snipe or slash them as you prefer, though note the oil-jar/hay-mat near the entrance make a dandy ambush spot.
When they're dead, loot the carbon crystals from a mat next to small bench in the back room to complete the location.