Slab

Slabs, apart from being concrete blocks used by Humans, were also robotic beings - typically androids, - used by various races, typically criminals, as basic slave drones who tended to travel in twos. They had a vague form as to not provoke suspicion, but their attitude and internal anatomy were very different - they obeyed the commands of their master, they disrespected other beings, did not speak and had the same composition all the way through. For example, Slabs used by a Plasmavore on Earth (and, due to the Judoon's intervention, on the Moon as well) looked like motorcycle couriers dressed in black, but never delivered anything and were composed entirely of leather, exept for the helmets.

They could be "killed" by sufficient radiation, (5000% more than an average hospital X-Ray scanner would use) although the same amount of röntgen radiation was ineffective against a Time Lord. In the words of the Doctor on how harmless it was to him, "[The Time Lords] used to play with röntgen bricks in playschool!" (DW: Smith and Jones)