Crystals are valued both in Daventry and in the Other World for their beauty, usefulness, magical, and other properties. Both table salt and sugar are crystalline substances, for example. Lasers make use of crystals in order to help focus and concentrate their beams of coherent light, and jewelry makes use of its crystals in order to focus and attract romantic energy. And, it is well known what a "girl's best friend" is, diamonds.
Many in the other world attribute healthful, curative, and protective properties to crystals, although conventional scientific thought is quite skeptical about such claims. Nonetheless, people in Daventry are quite skeptical about the laws of science, and what they refer to as "...dread technology." They make daily use of crystal's magical properties in all of its forms--amulets, stones of power, scrying stones, magical crystals, and powders of various powers.
Crystals are also used to protect against magic, and physical danger or attack. Held or manipulated properly, certain crystals reflect attacks back at their casters causing them to suffer the effects of their own aggression. These powerful crystals, however, are not common, and are most often found clustered in caves, both in the high mountains and underground. In Daventry, this means there is a dragon or yeti or other monster around, and this helps account for their scarcity.
The Yeti Cave (aka Crystal Cave) is filled with green quartz pegmatite and ice crystals. The ice crystals and others act like a concentrator of electrical energy-first collecting and storing it, and then zapping it back at the source.