Warp star

"It`s an explosion, Mickey. An explosion waiting to happen."

A Warp star was a small pendant shaped device which contained enough energies within its small form to create a powerful explosion.

Information
Sarah Jane Smith was given a warp star by a Verron soothsayer and after she escaped the Daleks on the Crucible, she informed Jack Harkness that she possessed it. They planned on detonating the device within the New Dalek Empire's space station, thus crippling it. However, the Daleks used a Transmat beam to quickly remove the threat by capturing Sarah Jane and Jack, as well as Jackie Tyler and Mickey Smith. (DW: Journey's End)

Behind the Scenes

 * Five different Warpstar designs were originally presented. This design was picked as it reflected Sarah Jane's character.