They're also called crystals. I bought a couple from the local Psychic store when I had that manifestation problem (it was a giant mosquito-like insect on my chest, if anyone's curious. It's still there, but got much smaller after I figured out what it was). To use them you have to decharge/purify them, and then channel energy into them. From what I understand, they take the enrgy you put in and sort of "convert" it, changing it's shape and frequency and all that to make the energy useful for a specific purpose. I'm not sure on how effective it is, or if it's even "for real", and it may well simply be the symbolism effect. The one's I know off the top of my head are Malachite, which is supposed to bring prosperity/riches; Jet, which is supposed to ward off malicious threats; and amethyst, which is for contemplation.