It probably has and I'm clutching at straws but in the short story 'Days Gone By, Days Yet to Come', isn't the crystal Azem leaves with Hythlo the thing that ends up becoming the key?
"Thus spoke Hythlodaeus, having ambled into his office at the Bureau of the Architect to find a crystal sitting rather conspicuously on a chair reserved for guests. Slipped from a singularly adventurous pocket, no doubt. He marveled at the way the light played on its facets, like the surface of a tranquil lake, and recalled the first visitor he had received that morning.
Azem had sought the chief's assistance with the refinement of the fourteenth in his series of concepts to help ease the burdens of travel, which was contained within the palm-sized crystal."
I mean, it sounds pretty close? Unless I'm wrong and it's something completely different.