To my understanding it is pulverized rock and calcareous marine matter... Can't recall the last time I've seen similar.:-)They are fantastic!Also I should say that the burnished calf is beautiful. Soft, supple, and with a really fine grain that makes the creasing subtle.I don't think we see much made up in it in this thread. Probably viewed as too dressy or not durable enough. But it is super classy.