A crystal ball is a magical device that allows its user to scry (i.e. view remotely) on a person or place as long as said person or place is not shielded from scrying. This scrying is capable of crossing the borders of both the Sailor Moon and Dungeon Keeper universes. Especially enchanted objects such as amulets can detect when a person or location is being scryed on, as can trained individuals such as Sailor Mars or her grandfather.
Operating a crystal ball requires a certain constant amount of magical energy and can be taxing on the magically weak or untrained person, especially when crossing dimensional boundaries.
The limitations on zooming in with a crystal ball are unknown, though at least a microscopic view on blood cells and bacteria is possible.
Two crystal balls can be linked. Linked crystal balls are limited in their view to that of their partner, but in exchange receive audio as well. The link is temporary and can be broken by both parties at will.