Aetherium Brick

The brick lights itself, and that is the practical reason to use it. A room floored or walled in this needs no torches at all, and the light is flat and even rather than pointed, so you avoid the bright spots and hard shadows that lamps leave on a wall.

It also throws off little sparks of star dust when you stand on it and leaves a trail behind you when you run. That is charming in a corridor and distracting in a room where you are trying to read chest labels, so pathways and feature walls suit it better than whole interiors.

The useful part is the timing. You can have this before Hardmode, which puts a self lighting building material in your hands earlier than most people expect to get one.