What material is used in a stylus?

Hi everybody,

i'm working an a small project at my university - one part is building a stylus but we're totally stuck in finding out what materials are used.

Lets say i have an aluminium-core, what rubber has to be on the tip of the stylus/pen to trigger a "touch-event". I mean, we not only need to know what exact rubber, we need to know what exact property/characteristic of that rubber makes it conductive/touch-friendly.

I'd really appreciate if you guys could help us out. We're searching for weeks allready :-/