Gravatite is the rarest ore in the Aether. It can only be found on the largest floating islands, and mining

down in a cave to find it. It has a purple appearance.

Since it can only be found on floating islands, this ore can be hazardous to mine since you might fall through the island if you mine through the bottom. Try mining from the bottom first, it's quite hard to fall upwards. But this ore does fall upwards! So it's a better idea to mine it from underneath, so that's two hazards ticked off.