Amalgam Stone  

I've only ever heard of them dropping from epic raid mobs but then again some people sell the item over the broker for 2-78 platinum... varies greatly in cost.

Also, in addition to Tremblar the behemoth it has been rumoured that Vaz'Gok the Cursed(http://eq2.ogaming.com/db/bestiary/VazGoktheCursed.php) drop such fabled materials. My guild killed him last night but didn't get anything like that but I doubt it drops every time.