Crystals can have a huge effect on us. The energy they give off can change the way we feel, or think, or are seen. As TDS said, selenite is great for pain relief, and it never left his sight.
For example, if you purchase a crystal for love (such as Rose Quartz), and carry it around with you, the energy it gives off will attract loving vibes. Of course, it could be for self-love, not just for relationships with other people. And of course, you can get many other uses. Clear Quartz can be empowered with the properties of any other crystal.
I recently purchased a stone which had calmness as one of its metaphysical properties. Its calming aura mixes with our bio-eletrical field and influences us.
And yes, there is some that can enhance your psychic abilities, such as Amethyst, Aquamarine, Azurite, Citrine, Lapis Lazuli, Sugilite.
A truly great site on crystals with info, although not many crystals are listed.http://www.wicca-chat.com/stonesandcrystals.htm
Another great site with lots of info and lots of gems/crystals listed:http://www.shimmerlings.com/gemstones.htm. I suggest reading most of the links under the picture. :)
Please also note that the bigger the gemstone, the larger the amount of energy it gives off. And how pure it is (not chemically altered) can also change how well it preforms. Make sure you cleanse your crystals before use.