There are a number of different types of crystals, each filled with their own healing abilities for the mind, body, and soul. They're thought to promote the flow of good energy and help rid the body and mind of negative energy for physical and emotional benefits.
Positive crystals (Opt) a doubly refracting crystal in which the index of refraction for the extraordinary ray is greater than for the ordinary ray, and the former is refracted nearer to the axis than the latter, as quartz and ice; -- opposed to negative crystal, or one in which this characteristic is reversed, as Iceland spar, tourmaline, etc.