Trionic lattice

A Trionic lattice or Time Key was a small element essential to the workings of a time cabinet.

History
A Trionic lattice took the form of a small, very fragile, crystalline disk. If it fell to the ground, it would shatter into a thousand pieces. It apparently would also light up in the vicinity of the time cabinet, allowing one to locate it.

Each Trionic lattice was unique, and therefore could be used as an irreproducible key to the time cabinet.

Magnus Greel used his Trionic lattice to locate his lost time cabinet. The Fourth Doctor shattered it, preventing him from using it. (TV: The Talons of Weng-Chiang)