Minecraft wiki entry for Glowstone, associated text: When placing redstone dust on a glowstone block, being a transparent block, signals can pass from one block to another diagonally. Redstone wire, even though it may appear to connect up the side of a glowstone block and to the wire portion on top, does not send a redstone signal down the glowstone block. This makes glowstone effective for vertical diodes, logic gate designs, and space-efficient instant vertical redstone.