Some Thiel CS-6s had concrete front baffles. Magico uses 6061 aluminum in their speakers. I have a pair of Green Mountain Audio Europas in my room right now and they seem extremely heavy, dense, and inert.
Speakers made with unusual cabinet material
There were the Celestions with their Aerolam cabinets. The Green Mountain Audio speakers that had a sort of cast marble cabinet. Were/are there other speaker manufacturers that used some kind of odd or exotic materials for the cabinets? Are there speakers with real wood cabinets (rather than veneer?)