Crystal-gazing (also known as crystal-seeing, crystallism, crystallomancy, gastromancy, and spheromancy) is a form of divination or scrying achieved through trance induction by means of gazing at a crystal.
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." 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.