Magmatic Stone  

Magmatic Stone is an object - click your Ember Mining Pick to open it. Drops a Emberheart Shards roughly half the time. The drop does not show up in the advanced loot window, you have to loot them the old-fashioned way (with corpses visible).

Magmatic Stone is trackable. There are always 8 up at a time. When you open one, another appears elsewhere in the mines within a few seconds. Usually they're sitting on the ground, but can sometimes be found hiding inside a mining cart.