Taken directly from the Kaweco Catalogue:
As early as 1905 Kaweco had already manufactured the first writing instruments made out of metal. Products which were made out silver or gold plated brass in the past, are made from aluminium today.
High-quality aluminium is processed by CNC-technique and other processes in order to obtain a satin finish and hardened surface. The “Stone-washed” version already has a used look. The “Raw” model develops a patina especially quick because the aluminium is untreated.