...How about canvas? It fits with the whole 'adamantine=circus' bit.
...That's ridiculous.
I never really understood the idea of making a big hush-hush spoilery
thing of the demons and adamantine. For starters, they're deep enough as to almost never appear unless you're actually looking for them (very rarely: exploratory mining for other metals if your fortress has actually managed to go long enough as to exhaust the upper levels). On top of that, Dwarves occasionally have preferences for one or the other. "Corrupt Intentions" and "terrifying features," anyone?
Plus, the only thing accomplished by fussing over what we're calling it, is causing the uninitiated to grow curious and search about the wiki for it. Spoilering by proxy, if you will.