I love crystals! Rather than using them to "guide" my dreams, I use them to keep the energies clear so they can come through without negative interference.
Lauren de Silva, New Age editor here at Bellaonline, has some fantastic articles on specific crystals. She even offers a crystal course which I am taking. The first class is free.
Everyone should use what feels right for her. I personally use water which, although it is not a crystal, is a potent conduit for me in its many forms. I am a water element.
But I always seem to keep rose quartz nearby as love really is the highest force. Flourite is said to clear negative energies and thus should be kept in every room of the house!
Amethyst provides a vast array of helpful properties. Extremely powerful crystal. One property is that it aids pleasant dreaming so I encourage dreamers to keep an amethyst gemstone under their pillows.
I wear amethyst often, especially during meditation.
Lapiz azuli and Sodalite also aid dreaming. Citrine and Ruby can prevent nightmares.