
Scrying is the art of divining by looking into an appropriate surface. It could be water, a mirror on the wall, a crystal ball, or a slap of rock. For that matter, some people are quite talented at seeing visions in the flames of fire or in the bottom of a teacup. Smooth, natural surfaces are much better and less distracting, however.
I like to think that a chunk of shiny black obsidian was the first scrying mirror. We know the ancients had special prophets and priestesses who engaged in foretelling the future, and they were making and using tools of their trade from various crystals at hand. No doubt they would be delighted to know we are still using crystal balls made from translucent quartz and mysterious volcanic obsidian! I cleanse mine before and after each use with rainwater that has set through at least one day of sun and one night of moonlight, but rituals can get as elaborate as you want!
...