Effigium Ore

Effigium Ore is a tier 5 ore found on Lightless planets.

This ore is completely invisible, so you require an ore detector to mine it efficiently. (some of the options: Multi-ore Detector, or Precision Ore Detector, or robots like Ore Drone II and Ore Drone III)

Alternatively, just fire a highly upgraded Mining Laser in all directions in hope of hitting the ore that you can't see.

Tips
A common mistake is to seek Effigium in subbiomes (not in the main biome) of the Lightless planet. Effigium will exist in Lightless or Lightless Cave biomes, but subbiomes have their own ores (and most likely won't have any Effigium).