"crystal ball" for scrying/fortune telling/prophesy. The traditional "hollywood sphere is quartz; and that indeed works well; however some famous mediums have used amethyst and obsidian, preferring those over quartz for a better purity and resonance, but it's a personal preference. The scry-er enters a half-trance state and stares apparently at but honestly deep into, beyond physically seeing, the sphere, until they begin to see/sense images, either literally on the surface of the ball or in their minds.