Chalcedony
Chalcedony is a type of quartz used by the ancient Egyptians to drive away ghosts, night visions and sadness. Chalcedony is never crystalline, but translucent and waxy in appearance. It usually is a smoky blue, but it may also be yellow or a cloudy white. The Egyptians used it on their scarab seals.
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SOURCE:
The Encyclopedia of Ghosts and Spirits – Written by Rosemary Ellen Guiley – September 1, 2007