Extra heavy glass. Good quality. Joint would be better if it was ground.
So I've had this rig for quite a while now, and something you should know is that I'm TERRIBLE with glass. I always break it one way or another. I'm incredibly too clumsy, so having a piece stick with me this long is insane. I had it for over a year before I finally did it in. I immediately ordered a new one because this rig is durable, functional, cute, and easy to travel with. Highly recommended!