Margaret LeFay uses a jewel to impart knowledge when Harry Dresden has a vision during a soulgaze with Thomas Raith. Though Dresden doesn't receive a physical jewel, LeFay uses it to give Dresden insight in the vision.
LeFay uses the same technique to record voice messages of The Ways she was exploring on a red ruby. 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 the Leanansidhe to be given to Dresden when he was ready. It is cut to fit into the socket of Dresden's pentagram necklace inherited from his mother.
In the series
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. LeFay calls it Insight, or knowledge. And when she passed it to Dresden, he saw a flash and felt a tingling pain in his head. Dresden 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.
In Changes, Harry Dresden 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. Dresden took his team with him: Susan, Karrin Murphy, Thomas Raith, Sanya, Molly Carpenter, Mouse, the Leanansidhe , Martin.