The Crescent Moon of Llyr was an enchanted silver amulet, worn on a silver chain as a pendant by daughters of the House of Llyr. Angharad Daughter of Regat had one set with an enchanted jewel of the Fair Folk. Her daughter, Princess Eilonwy, wore a similar but apparently non-magical crescent throughout the Chronicles of Prydain.

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History and Powers of Angharad's Pendant

The jewel that adorned the horns of Angharad's crescent was "strangely carved" and "clear as water". Its facets sparkled as though lit with an inner fire.

Doli of the Fair Folk told Taran that his people had given then-Princess Regat the gemstone as a wedding present, and surmised that Regat had handed it down to her own daughter, Angharad. Years later, widowed and exiled from her people, Angharad sought her kidnapped daughter Eilonwy; while searching she came upon the wizard Morda and, out of desperation, offered the heartless recluse her pendant and enchanted jewel. She explained to him that the jewel could "lighten burdens and ease harsh tasks"; Morda employed it, after years of study and practice with the magical gemstone, to raise the wall of thorns encircling his abode, to reduce timber to twigs, and -- most surprising of all -- to find the hidden way posts and treasure houses of the Fair Folk.
During the events of Taran Wanderer, Morda also used the jewel to transform a Fair Folk spy into a mole, Doli into a frog, Fflewddur Fflam into a hare, and Gurgi into a mouse. After Taran defeated the hateful sorcerer, he gave the jewel to Doli, who offered deep gratitude and promised to return in to King Eiddileg, as it "was too dangerous in other hands". Doli opined that this was the greatest service any human had ever done for the Fair Folk.
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