Temporal link

A temporal link was a portable Time Lord device resembling a medallion or amulet with a blue gem in its center and inscribed with runes. The Fourth Doctor stole one from the black collection in the Prydonian Academy on Gallifrey while he was Lord President. During a visit to St. Christopher's Church in Cheldon Bonniface, the Doctor hid the temporal link in the brickwork of the church, believing that it might come in useful one day. On 20th December 1992, the Doctor retrieved the link and handed it to Emily and Peter Hutchings before going inside his own mind to battle the Timewyrm. While there, he sent a telepathic message to Saul which also activated the link, turning it into a doorway into the Time Vortex. Peter worked with Saul to calculate a passage to the Doctor's mind, enabling Emily to enter it and rescue the Doctor's consciousness. (NA: Timewyrm: Revelation)