The Gemcutter was a prototype device developed by Carbanti United Electronics that counteracted the cloaking effect of ships in hyperspace, allowing the user to be aware of where a ship would drop out of hyperspace, usually only five seconds before, but as one user mused, "Those seconds could mean the difference between life and death."
The prototype was stolen by the Dark Jedi Asajj Ventress, who installed the device in her deadly Ginivex-class starfighter, known as the Last Call.