This gemstone was the most significant possession of the Trolls, for it is a fundamental marker of their relationship with the Dreaming. It was used in the creation of the Sluagh. Lady Eiluned gained the emerald through trickery in payment for Boggan's service to the Trolls. Without it the reincarnated Mountain King could not prevent or lessen the terrible consequences of the Trolls' Oathbreaking.
The emerald acts as a focus for three fundamental powers of the Dreaming: the Trolls (the rules of the Dreaming), the Sidhe (rulership), and Dragons (freedom).
The emerald is best thought of, in OOC terms, as a metaphor for the Trolls' wisdom, of their link with Dán.