Minecraft wiki entry for Redstone_components, associated text: Redstone dust or a fence gate can be attached or placed on the top of glowstone, and a trapdoor can be attached to the side of glowstone. Redstone dust on top of glowstone can transmit power to redstone dust adjacent to it or diagonally upward, but not diagonally downward. Because glowstone is not opaque, it cannot power an adjacent block (including an attached trapdoor), but redstone dust on top of it can.