Intro to Tarot page 219-:

Clear Quartz:  brings clarity

Dark Stones:  such as smoky quartz crystal or obsidian can keep you centred if you become dizzy or flighty while doing a tarot reading

Rose Quartz:  helps to open your heart for compassion

Amethyst:  crystal aids in opening the third eye to develop clear psychic vision.  If you use just one stone, amethyst may be the best choice overall.