From what I've read it's a carbon and aluminum composite. I just found this description of one.

RAC stands for Reverse Advanced Composite and represents the most advanced carbon composite suspension fork ever developed for a mountain bike. With nearly two years of co-development time between Marzocchi and Easton Composites, the RAC is ready to go. Using an upper structure made from carbon and aluminum composite; the RAC has the most rigid crown and leg assembly ever created. This rigid construction gives a perfect, flex-free assembly for the inner legs making stiction non-existent and providing phenomenal small bump performance. The RAC's lower section uses 20mm magnesium dropouts and 30mm Easton inner legs to keep things nice and stiff during sessions of great stress.