Silversmith's mirror

The Silversmith's Mirrors were a set of mirrors charged with energy “not of this world” (in the Eighth Doctor's words). Unbeknownst to the Silversmith, the 19th century Scottish magician who had crafted them, they were connected to a pocket of unreality thanks to the presence of Spherion crystals inside said bubble dimension. They could, therefore, be used as portals between Edinburgh and this pocket world, allowing the Mirror Folk to break out into the wider world so long as their human originals were trapped inside the pocket dimension.

As exploited by the Mirror Silversmith once he began posing as the original Silversmith and performing magic shows in his stead, it was also possible to step into one mirror and step out of the next a split-second later, giving the appearance of teleportation. The Mirrors were eventually shattered by the Doctor using his sonic. (COMIC: The Silvering)