Glowing effect on foreground blocks: Mud (top) and Brick (bottom) both with Diamond Gemspark Walls behind them (left) vs no walls (right).
Gemspark Walls are a group of walls made from Gemspark Blocks. The Offline Gemspark Walls are a darker variant of them. Unlike Gemspark Blocks they seem to have no mechanical function but they glow the same way as the Gemspark Blocks. The walls can't emit light onto adjacent blocks or objects, however, they do cause blocks placed over them to appear lit.