Delrex is a plastic that is used to replace tortoiseshell since the trade of tortoiseshell was banned in the late 1970s. Delrex is essentially the same as tortex, only under a different name. It was created as an alternative to tortex, a plastic used by Jim Dunlop.
Delrex is used as the material for Dunlop's "gator" picks. The tortoise series are also made from Delrin but with a different surface.
D'Andrea also used Delrex picks.
References
Delrex Wikipedia(Text) CC BY-SA