After the defeat of the Horned King, Gwydion gave a gift to each of the Companions, observing: "These are small gifts for great valor." Eilonwy was the fourth (of five) to receive her present, to which the Son of Don added: "It is precious; but to me her friendship is even more precious."
The gemstone was "carved by the ancient craftsmen of the Fair Folk". Orddu would later observe that the ring was as pretty as Eilonwy herself, though much older.
End of an Age
Eilonwy, when faced with the choice to accompany the Sons of Don and all those who could wield enchanted powers to the Summer Country -- or else remain in Prydain with Taran -- chose the latter. To wish away her powers of enchantment, she employed the ring. "Turn the ring once upon your finger," Dallben told her, "and wish with all your heart...." The Princess winced when the ring flared, but she felt not a whit differently after her powers were gone.