In the X-Men series, there is a helmet which can block a telepath's ability to read somebody's mind. That helmet is made up of what material?
In the comic canon, Magneto's helmet is made out of whatever metals Magneto has available. Any strong metal appears to be usable.
He reshapes the metal at the molecular level so it has the anti-psionic abilities necessary to keep his mind from being or affected by telepathic abilities. The metal is likely highly magnetized, since strong magnetic fields reduce or prevent telepaths from being able to read minds. See: Faraday Cage
Note: he does not need the helmet to protect his mind from being read. An active use of his power can do the same thing. The helmet allows him to not focus any effort to protecting his mind and gives an added layer of protection against Alpha level telepaths such as Jean Grey or Professor X.
We have watched him shape his helmet, even when it is lost or damaged out of scrap metal if necessary. Magneto does not appear to be concerned about losing a helmet and always seems to have a new one, even it it is captured or destroyed.