Worldbuilding and biology have always been two huge passions of mine, especially when it comes to dragons. So well, at the moment I'm working on a little project in which I'm attempting to make different types of dragons more biologically plausible, and one type in particular really frustrates me. I've seen dragons and other creatures depicted swallowing rocks and then spitting out molten lava, an easy example that comes to mind is the Gronckle from the How to Train your Dragon franchise.
I was wondering, is there any way to make such an ability plausible? Like having the creature's stomach digest specific minerals of the rock in a furnace-like stomach without suffering damage from the heat, and spit them out half molten. If not I was also trying to think of alternatives to such ability that would fool humans into believing the dragon is spitting out molten rock, when it's actually something else. The only thing coming to mind in that case is projectile vomiting, but honestly that's rather lame to me and not something I see sparking myths about beasts spitting lava.
Edit: After much brainstorming from the comments I think I came up with an possibility.
The dragon could store rocks in a pouch which produces an oily flammable liquid. Strong muscles would contract the pouch to projectile the rocks out of the mouth like a rain of debris.
There would be two orifices on the dragon's lower jaw which spray a catalyst substance as soon as the rocks come out. The catalyst reacts with the oily coating and lights it up on fire.
Since the flammable liquid is oily it won't mix with the thick, mucous coating of saliva protecting the insides of the mouth. That makes the saliva more effective as a protection from the flames and prevents it from making the rocks less flammable.
The catalyst spray would probably cause a small burst of flames in front of the dragon's mouth, which only adds to the look of erupting lava. The nostrils would also close so the fumes don't go into the respiratory system.