Kessel

Spice was mined on Kessel by criminals. Since Kessel lay within the Maw Cluster, if a prisoner were to escape (very unlikely under Imperial rule) they would not likely go far because of the black holes. The spice found on Kessel was glitterstim, one of the most prized spices in the galaxy. This particular spice was created by a species of energy spiders that would spin the spice into webs. Any unlucky miner that came into contact with one of these creatures would have their life energy sucked out of them in seconds.

Spice mining was a very dangerous job. Kessel had no natural atmosphere and factories on the surface create air; however, the air was still too thin to breathe without an oxygen mask. Spice was activated by light, forcing workers to work in complete darkness or under dim, red lights that lacked photons energetic enough to trigger it. Along with the obvious dangers of energy spiders, glitterstim threads were very sharp, and could easily remove digits if the worker was not careful.

Characters on Kessel

Currently Arya Ravenwing, Jaassuuvi Ageerrodarr, and Callomas Savoc are all imprisoned on Kessel, for various crimes against the Empire. (link)