Shan's Ring

a brass ring with a non-reflective round black stone given to Shan by the unicorns

stored on the fourth floor of the West Tower; Laura says it was in a lead casket in a niche on the north wall but the children find it embedded in the floor

its purpose is described by the Riddle of Shan's Ring

"I am a trinket in the world,

Unvalued gold and sullen stone;

But outside power is unfurled,

When outside Power I am hurled:

Then Time awry is blown.

What am I?"