Telos is a barren planet which was conquered by the Cybermen. Though it was originally inhabitated by the Cryons, they moved to underneath the surface when it became too warm. The Cybermen later came to steal the referigerated cities to use for suspended animation, killing most of the Cryons in the process. The Cybermen used Human slaves to mine the planet, but they were stopped when their base was blown up.
Telos is a grey and barren, but diamonds, vastial and other minerals can be found on the planet. (DW: Attack of the Cybermen)