The Golden Tongue is an essence gem which grants a moderate bonus to all three conversation skills (Charm, Intuition, and Intimidation). It is the strongest gem in a series of three, the other being The Bronze Tongue and The Silver Tongue.
It can be purchased from Zin Bu at any time after reaching Chapter 3.
In-game description: "The treacherous advisor to the great Emperor Sun Jin ordered the Order of the Lotus to construct this gem. The gem gave the advisor nearly unparalleled manipulative power, but in the end the powerful Emperor's own skills of persuasion put his ambitious adviser in his place."