> What is the most environmentally friendly analog writing tool?

I don't know how practical it is, but the wax tablet[1][2] is perhaps the closest thing meeting this description that I've seen currently available [that I can think of, anyway]. No personal endorsement or any agreements or experience one way or the other with the seller, just something I found and thought was interesting.

[1] https://www.scribalworkshop.com/shop/historic-wax-tablet

[2] https://www.scribalworkshop.com/blog/2019/6/5/how-to-use-a-w...

Write on vast amounts of acid-free paper, which will happily sequester some carbon for a thousand years.

Blood? It's renewable.