Because crystal gazing has been developed by people of various cultures through a long period of time, the term crystal gazing denotes several different forms of a variety of objects, and there are several schools of thought as to the sources of the visions seen in the crystal gazing trance.
Crystal gazing may be used by practitioners -- sometimes called "readers" or "seers" -- for a variety of purposes, including to predict distant or future events, to give character analyses, for fortune telling, or to help a client make choices about current situations and problems.
With respect to the tool or object used to induce the crystal-gazer's trance, this can be achieved with any shiny object, including a crystalline gem stone or a convex mirror -- but in common practice a crystal ball is most often used. The size of ball preferred varies greatly among those who practice crystallomancy. Some gazers use a "palm ball" of a few inches diameter that is held in the hand; others prefer a larger ball mounted on a stand -- although most authors agree that the expense of a very large ball is not always justified by added efficacy. The stereotypical image of a gypsy woman wearing a headscarf and telling fortunes for her clients by means of a very large crystal ball is widely depicted in the media and can be found in hundreds of popular books, advertising pages, and films of the 19th, 20th, and 21st centuries, and the pervasiveness of this image may have led to the increased use of fairly large crystal balls by those who can afford them.
Books of instruction in the art of crystal gazing often suggest that the ball used should be perfectly spherical (that is, without a flat bottom) and should be supported in a wooden or metal stand. If made of glass (e.g. lead crystal), it should be free from air bubbles but may be coloured. If carved from natural crystalline stone (such as quartz, beryl, calcite, obsidian, or amethyst, it may display the natural colouring and structure of the mineral from which it was fashioned. Some authors advise students to place a sigil, seal, or talsimanic emblem beneath a clear sphere, but most do not. Most authors suggest that the work of crystal gazing should be undertaken in a dimly-lit and quiet room, so as to foster visions and more easily allow the onset of a trance state.
As for the origination of the trance visions themselves, some practitioners claim that crystal gazing engenders visionary experiences and preternatural and/or supernatural insight, while others think that the visions arise from the subconscious mind of the crystal gazer. Interestingly, some authors accept both positions as not mutually incompatible.
Some stage magicians use a crystal ball as a prop and crystallomancy as a line of patter in the performance of mentalism effects. This type of presentation is sometimes referred to as a "C. G. act" - "C.G." standing for "crystal gazing." Perhaps the most famous expositor of the C. G. act during the 20th century was Alexander The Crystal Seer, billed as "The Man Who Knows All." Another stage magician and mentalist who was also a crystal gazer was Julius Zancig, but he did not perform a C.G. act in public -- rather, he used the crystal ball in his work as a spiritual counsellor for private clients.