Heavy duty borosilicate glass! Great price! I couldn't stomach paying 60 bucks plus shipping, for a tiny piece of glass from Focus V that's made in China anyway, because it's a name brand. Very similar in size to the original basic glass that comes with your Carta. And delivered to my mailbox in 8 days from ordering. My only critique would be to widen the opening on the base that mates up to the Carta silicone opening. I'll be ordering the whip attachment next.