Not sure exactly ~what~ type of insulator this is or what it's made of, but it "tinks", with the exact same tone that glass does when I hit it with my ring. If it's glass, it's crude and true black glass. No light passes thru this insulator and I'm not about to give it the bounce test to see what it's made of. Surface of the insulator is another matter, it's scratchy grey black and looks like it was from factory that way or it was in a sand storm but it really does look more factory application. Looks to be foreign and VVNM condition. Is embossed B.M.P / 49 on the dometop. $20 + shipping for this "thing".