Minecraft wiki entry for Mob_grinder, associated text: Although magma blocks do not destroy items, collecting the items can be a little tricky. There are generally two options for item collection: Use water streams to wash items off the magma blocks or use hopper minecarts. Water streams have the downside of negating any fall damage and potentially causing the mobs to bob up and down, taking damage only some of the time. Running hopper minecarts on rails underneath the magma can solve both of these problems but is more expensive.