The polished glass wall would be crafted with 1 glass wall at water or a sink. It has the property of being perfectly clear/invisible. Because I'm not evil, it does not have a block equivalent. For convenience of placing, it will appear with a red tint while holding polished glass walls. The reason behind this idea is that mobs do not spawn in front of player placed walls, giving a way to pacify an area without ruining its looks.