It's carefully machined strong aluminium. A litle expensive but you'll regret not buying this if you crack the die (very common).
EXCELLENT. My Asus P8z77-VLK 3750k cpu socket and cooler needed tiny modification, but every thing fits fine and works. 10 to 20C drop in temps. Now I can overclock to stable 4,8ghz@1,55V with aio. I recommend.