Minecraft wiki entry for Transmission, associated text: Upside-down slabs are the transparent block most commonly used for this purpose, but glowstone is used where light would be useful (to suppress mob spawning, etc.), upside-down stairs can be used where a full-side solid surface is required without light (for example, alongside a water channel transporting items over ice), and hoppers may be used in this way where they are already being used for item transport.