Bourse Resources
Three materials are “too bulky to easily transport to Anvil”, so “the Imperial civil service provide Bourse certificates for every wain” of Mithril, Weirwood, and White Granite.
Bourse certificates resemble large banknotes; there have been several printings, and Bourse certificates in which the Throne is matte black are just as valid as Bourse certificates in which the Throne is sparkly and silver. When these are handed in, return them to the big box. You might, very occasionally, have players hand in very old, very battered Bourse certificates printed on vellum. These should be treated as valid, but instead of returning them to the main stock of Bourse resources, rip them up and put them in the bin.