These caps are made from Aluminum and have been anodized.
The newest edition is made from Surgical Grade Stainless Steel (SS 316L)
The caps fit very tightly on the Pyrex tubes, with a nice firm seal.
Likewise, the inner o-ring has a good solid grip on the cartomizer.
You can choose the color of the o-rings.
The Black is made from food grade EPDM.
The Red is made from food grade Silicone.
I advise that you leave a drip tip on your cartomizer or use one of the tank filling tools sold here or by other vendors when inserting it through the end caps to prevent tearing of the o-ring.