The Astral Conclave

This necklace consists of nine interlocking rings of dark mithral surrounding a star sapphire that contains a miniature, endlessly turning spiral of light. The rings never quite touch. They rotate in different directions whenever the wearer prepares or casts a spell, and the sapphire briefly displays the sigil of every spell it has ever contained. The necklace feels weightless while worn, but becomes impossibly heavy to any creature that is not attuned to it.