Trionic lattice

From TARDIS Index File, the free Doctor Who reference.

A Trionic Lattice or Time Key was a small element essential in the workings of a time cabinet. It took the form of a small crystalline disk, and was very fragile, if it felt to the ground it would shatter in a thousand pieces. It apparently would also light up when in the vicinity of the time cabinet, allowing one to locate it.

Each Trionic Lattice was unique, and therefore could be used as unreproducible key to the time cabinet. (DW: The Talons of Weng-Chiang)