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Cosmetic prismatic pendant was removed from RuneScape after an update, but still exists in-game for those who had obtained it.
[FAQ] • [doc]

A cosmetic prismatic pendant is an uncharged prismatic pendant. On 1 December 2014, all cosmetic Pendants of Skill held by Diango or in a POH toy box were changed into a single cosmetic prismatic pendant. Players must select a skill before they can equip the pendant, turning it once again into a cosmetic Pendant of Skill. They may also change the skill without destroying it first.