Minecraft wiki entry for Glowstone, associated text: Glowstone blocks all sunlight, players and mobs suffocate inside it, mobs are able to pathfind to it, and it blocks a beacon beam, preventing the beacon's activation. However, like transparent blocks, it does not prevent the opening of chests, monsters (including slimes) cannot spawn on it, and it does not transfer a redstone signal. This behavior also occurs in sea lanterns.