Volcanic Veins

Would be one of feature within caves, it features lava coming from the walls and high probability of faults with lava everywhere at their bottom. Clusters of Coal Blocks would naturally generate along fossils, basalt and lava blocks. Around those caves dungeons like to spawn. Also lava lakes.

This is called veins and not caves because it would look like magma scars running on the walls. And can generate in any cave subtype: stalagmite, water one, stalagmite ones, leafy ones and so on.