Lumite Ore is found within Lumite Nodes embedded in rocks underground in the lowest part of the Lava layer and also of course in the Corruption layer that is one giant cave with a top layer and bottom layer made from corrupted blocks. Lumite Nodes are almost never accessible through natural caves leading down from the surface, so you will need to dig mineshafts downwards yourself. Lumite Ore - like all ores - cannot be pulled from Nodes with your arctek-gauntlet nor any Power Cells equipped, it requires an Extractor of any kind to be harvested from Lumite Nodes.