Margaret LeFay's jewel

Margaret's jewel was a red jewel that contained Margaret LeFay's substantial knowledge of Ways through the Nevernever. It's first mentioned in Changes.

Description
The jewel appears as a red ruby, cut to fit into the pentagon socket of Harry Dresden's pentagram necklace, he inherited from his mother.

Margaret recorded voice messages of the ways while she was exploring. To use it, one has only to think of the destination, and if there was a way to it, the jewel would provide it, along with descriptions of the dangers related to it. The jewel was entrusted to the Leanansidhe to be given to Harry when he was ready.

In Blood Rites, during the Soulgaze vision with his brother Thomas Raith in which he sees and speaks with their mother, Margaret LeFay give him a small gem or jewel. Margaret calls it Insight, or knowledge. And when she passed it to Harry, he saw a flash and felt a tingling pain in his head. Harry comments that you don't gain knowledge without a little pain. There doesn't appear to be any actual jewel that he holds in his hand when he comes out of the Soulgaze.

Changes
Harry hurriedly and not too neatly attached the gem to his pentacle pendant which he also inherited from his mother. It fit perfectly. He made a scouting trip to the Red Court holding facility while Susan Rodriguez and Martin were still unconscious from the Leanansidhe 's spell. He used the gem to travel to the security holding in Nevada with Susan and Martin to break in to gain information on where his daughter was being held.

He then used it to travel to Chichén Itzá, Mexico to rescue his daughter. Harry took his team with him: Susan, Karrin Murphy, Thomas Raith, Sanya, Molly Carpenter, Mouse, the Leanansidhe, Martin.