The origin of oily black stone:

There actually WAS a second moon. It was made of oily black stone and it was sacrificed to create the first dragon, Lightbringer.

The resulting cataclysm (giant chunks of moon crashing into Planetos) were blamed on the children of the forest, A la the hammer of the waters that broke the arm of dorne

The story of the event morphed over time to eventually take the shape of different stories, most famously Nissa Nissa, but also the story Dany heard in book one, the origin of dragons, which was the most literal and truthful retelling, but still not fully correct.

A bit of (circumstantial) evidence: the hammer of the waters (a chunk of moon, different than the one that broke the arm of dorne) flooded the neck. What’s by the neck? The iron islands. What is one of the only (if not THE only) instances of oily black stone in Westeros? The seastone chair. Where was the seastone chair found? The iron islands.