Margaret LeFay's jewel

Margaret's Jewel was a jewel that contained margaret le faes substantial knowledge of Ways. It was intrusted to Leanansidhe to be given to Harry Dresden when he was ready.

Use
Margaret recorded voice messages of the ways while she was exploring. To use it one needs to think of where one wants to go and if the jewel knows a Way it will tell you. Recorded messages such as, " Beware of dangerous gasses such as..." would also be known.

In the series
The Jewel was intrusted to Leanansidhe to be given to Harry when he was ready. He used this to travel to Navada to gain information on where his daughter is. He then used it to travel to mexico to rescue his daughter.