120 Ranger. Slaughtered the whole southern part of the zone inside and outside the castle. I learned 3 things. 1). This is a Rare drop. 2) This has only ever dropped of a Doomfire Wizard and 3) Dropped off the Wizards in the castle, none outside of it. Hope this saves some frustration.
I need 9 of these for a combine in the Artisan's augment quest and I can say for a close-to-indisputable fact that these only seem to drop from Great and Greater Flame Lords, at least in the open field where the named frog, spider and flame lord all spawn. To get 9 of these I had to clear almost the entire field at least a dozen times as the drop rate is abysmal. But it appears to be ZERO or vanishingly close to it for any mobs other than these 2. All 9 of my drops came from these classes and I'm sure I killed at least 1000 spiders, horses, humanoids and other types, none of which dropped this item.
Don't worry about any other quest-related drops you'll need from here since by the time you see your Magma Crystals you'll have the opportunity to pile up stacks of the other ones.