I have never seen anything that looks even remotely like that that is not clay. I guess it could be something regional but it does look like you can see the red of the clay down inside the flue
Ha! Strange. It's certainly concrete, I can see tiny rocks in the mix. The construction of the blocks look very similar to concrete masonry units (cinderblocks) but made for a flue. I googled concrete chimney/flue liners and found a few photos of similar products but most were links to other countries. The house next door was built in the 1930's by the same builders (for the same original owners. this was their retirement home) and it has two clay liners poking out the top of chimney. There were a lot of custom (read funky) things about the way this house was built. Going to ask others from Oregon if concrete chimney liners are common to this area.