The LILIPUT ballpoint from Kaweco, made of copper, is particularly sustainable.  It is made entirely in Germany and thus goes through very short transport routes.  With daily use the copper develops a natural and unique patina.
 
Measuring a total length of just 98mm and including a high-quality cardioid curve mechanism, it sets itself apart from other ballpoints as a particularly precise and quiet writing instrument.
 
The body, made of high-quality stainless steel, gives this ball pen exceptional haptics and elegance.

The pen includes a blue 1mm refill that slides easily on paper.